You can’t reset it or simply “get a new PIN”. A passcode is mandatory so you had to have set one up to enable using TouchID.
Hopefully you have your databases on a Mac as well, or at a minimum synced in DEVONthink To Go as here is no recourse except deleting and reinstalling DEVONthink To Go.
The request is noted but bear in mind it would have to be an option for changing the PIN by first providing the existing one. Otherwise it would not be a secure mechanism.
iCloudKit worked for years now; but now, adding a new device, it seems that my stored security key is wrong.
I am willing to upload all databases to a new icloudkit location, but how do I get rid of the old one? When I delete the location and set up a new one, Devonthink tries to connect to the old location and tells me, the password is wrong. – There is one connection still working, so do I have to empty the location and delete it in Devonthink, or does it exist forever with my iCloud account?
There’s actually only iCloud (CloudKit) location, it’s not possible to set up multiple ones. In case of encryption issues due to a wrong key the location has to be cleaned and the databases uploaded again.