iCloud Sync invalid key

Dear all, I know this topics has been discussed already and I have followed your advice. However, my iCloud Sync doesn’t work properly so far. I run DTtg (2.7.7) on my iPad 2019 (iOS 13.4.1) and DT (3.0.4) on my Mac (OS 10.15.4). I have setup new sync databases, deleted the cache etc. My experience is that it first works fine but then I receive the “invalid key” message either on the Mac or on the iPad after. Is it due to conflicting documents?
Do you have any experience and/or advice? If not iCloud, which sync storage do you recommend. From what I have seen, Dropbox is no alternative, isn’t it?

Thank you for your help!!!
Cheers, CID

Are you able to successfully verify the sync store both on the Mac and on the iPad?

It is actually. And contrary to iCloud the synchronization is not asynchronous, the data is immediately up/downloaded and available on other devices afterwards.

Thank you for the prompt reply. I have tested Dropbox and it worked perfectly fine.