My Time Machine application doesn’t start backing up if DT is running. Only after starting up my Mac without opening DT, I can back my files. Is this a known error?
Thank you all!
Do you use encrypted databases?
Yes, exactly. My databases are encrypted.
Due to bugs of Catalina encrypted databases are excluded from backups while opened, otherwise Time Machine might unmount them although still in use.
Ok! Where is the right setting to configure the backup so that opened databases are ignored?
This setting can’t be configured, after closing the database it’s not excluded anymore.
O. M. G. - that is actually a vital piece of information; does this only affect Time Machine, or should I expect Arq and CCC to be equally affected?
It’s an issue with Time Machine.
The next release will include a different workaround which seems to be much more reliable and doesn’t affect Time Machine backups.
Thank you both again - I’m glad to hear that a) my backup strategy has not gone up in smoke and b) it’ll be restored to its full glory by the next release 
I have QRecall taking an hourly snapshot of all my databases. CCC once a day and TimeMachine.
But it seems to be good advice to quit DT3 once in a while. Thanks for that information!
I would LIKE this bug…
When Time Machine kicks off, it forcefully dismounts my two encrypted DT3 Databases for me. Log shows unexpected dismount. Workaround is to exclude the encrypted databases from time machine, but that leaves my databases not being backed up. Not ideal…
The next release will work around this issue.
Any guesstimates at a ship date? (All Software ship dates are tentative - slip happens!)
Definitely this summer.