As last resort you can mark all your data in you iOS as “download always”, then wait until device is fully sincrhonized and try the new store.
If you don’t have enough space in your iOS, do it by steps (folders or databases)…
If I understood the OP correctly, then they were getting a “wrong key” message on iOS too, so that wouldn’t work. Under those circumstances though, it might be helpful to better understand why after all.
@Niels just in case the whole thing was a fluke, I think I’d restart my iOS device and see whether I still get the error message in DTTG; if so, can you pls state when the message appears and post a screenshot? I find it weird that you were apparently able to sync the inbox to DT but then got an error message. Following reinstalling do you otherwise have access to iCloud (eg iCloud Drive) without error?
@cgrunenberg the OP mentioned that only the new/wrong key was to be found in the keychain.
If there’s a backup of the keychain, then it should contain the old key.
Where might one find such a backup on a newly installed Mac? 
Sorry, I assumed a backup was done at least before erasing the hard disc.
Exactly, I did a fresh install of macOS, signed up for iCloud and then installed DT3.
In DT3 I entered what I thought was the correct key for the iCloud sync which must have led to an updated key chain with the wrong key.
On my iOS devices I can see the previews of all content, but I am unable to download any files which have not been synced previously. A few of which of fully synced and available. (Not the stuff that’s really important…)
Am I screw*d? Any chances to be rescued?
Thanks everyone,
Niels
@eboehnisch Eric, can you help me with a question, pls? Please read the original (first) post in this thread. Does DTTG store the password for the sync store in the keychain? So would the fact that the keychain was updated with an incorrect password via a different device explain why the OP can no longer access the sync store from DTTG? Or is the password saved internally, which in this case would suggest the password of the sync store has changed (and, then presumably, the sync store is effectively empty)?
I ask, because in the first of those two possibilities the data is at least still there (if the OP can figure out the password).
@Niels do you have any other devices connected to you iCloud account which have been dormant since your mishap? A MacBook, for example? If so, you would have an old copy of keychain on that device, so long as you boot it whilst it has no internet connection (e.g. by turning of your WLAN). Or any old backup if your Mac?
great idea, unfortunately the answer is no. just one MacBook, an iPad and an iPhone, that ist always online.
I am about to give up, I have to say. And I can’t emphasize enough that it was 100% my fault. Usually, when setting up a new Mac, I copy the databases to an external hd and upload the databases to a cloud service additionally. Last time, I relied on the sync store as a “backup” and this time I did not even think about it. I could literally not sleep until 2:30 a.m. Good thing is: I have about 80% of the documents lying in a huge box, ready to be scanned once more.
So, if anybody from the DevonThink team could help, taking a look at my system remotely or whatever, I’d be willing to pay for that, really…
I’m really sorry to hear how you are feeling 
Whilst I’m waiting to hear back from Eric (and I’m not giving up until I’m sure what you are describing is a technically feasible explanation for why you can’t access sync store from DTTG) for future reference please allow me to point you to point you to this thread for ideas for a future backup strategy. If your data is in any way important, you really do need to have backups of it (I’m saying that in the kindest voice I can muster; I understand you are hurting just now).
It’s unclear from the previous comments:
Are you getting invalid encryption key messages in DEVONthink To Go?
I think that’s a yes, Jim. That was the basis of my question for Eric.
The optional keychain entry is just a backup, the sync prefers its internal stored values. Therefore another explanation might be that the Mac uploaded some data using the new (and wrong) key and when trying to access this data the error is logged.
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That was what I thought you might say; if that is the case, then the previous sync store is overwritten, is that right?
The sync code on Mac and iOS is identical and uses the keychain, if possible, and a fallback storage. But, actually, @cgrunenberg would be the better person to answer the details, if necessary. But, yes, the keychain could have the password.
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A sync store is never overwritten but it’s possible (not only theoretically) that different databases in the sync store use different encryption keys.
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Thank you very much, everybody, especially Blanc.
You are all very kind and very enthusiastic to help, that’s awesome.
What I will do now is rescue all possible files that were synced from my iPad and then start building the databases newly.Once that is done, I will start scanning the latest paper documents and bring some life back to my DEVONthink. What helps a little is the fact that I can still see which documents are missing because I have the previews and the files’ names still on my other devices…
But at first I will make up my mind about how to backup the databases both on an external harddrive and a cloud service…
Thanks again, have a great evening,
Niels
PS: it‘s just a Little frustrating, all the data is still in iCloud…
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But at first I will make up my mind about how to backup the databases both on an external harddrive and a cloud service…
We strongly advocate proper primary backups using TimeMachine (or similar snapshot-style backup applications) and external drives. Also, you should read the series of articles we posted: DEVONtechnologies | Search & Find
PS: it‘s just a Little frustrating, all the data is still in iCloud…
Yes, but it’s securely locked by the encryption key.
Is there a way to just try all possibilities I might have used as a key?
And is there a chance to access this built-in backup in DTTG?
You can edit the sync location and try different keys.
Tap the Edit link then the sync location and try as many times as you want.
And no, there is no way to access those backups.
Depending on how the Mac was erased, and how much data has been written to the drive since that point, there is a tiny chance that recovery software like Disk Drill could recover the original database from the Mac https://www.cleverfiles.com