In DT3 I replaced the folder’s “Created” date (which was suddenly 24.01.1984, I don’t know why!) with 25.03.2020 and the folder is available again in Finder!
The January 1984 date is the birthdate of the Mac, and you will see this on files and folders that in some way have been damaged or otherwise compromised. It might be worthwhile to check your drive to ensure there is no other damage.
I have had this same situation a couple of times when synchronising a DT3 (v3.8.3) between machines (direct connection on same LAN). It looks to me as though it happens if DT on the source machine is unable to fully access a folder (or perhaps file). In my case the folder was being created (by a finder copy and paste) on the source machine at the same time as DT3 was attempting to synchronise it with the target machine. I saw a popup on the source machine to say that database verification had failed and the log window shows several “Invalid File Integrity, checksum is different” messages. But once the folder is available again on the source machine the databases appear to verify OK. The only remaining problem is that the created date of the folder on the target machine is set to the Jan 1984 date. It doesn’t appear to cause a problem when viewing the files in DT but Finder will not open the folder. Changing the creation date (from within DT’s Get Info context menu) resolves the problem for Finder. Interestingly, I copied a large hierarchy of files and it was only the top level folder that had this created date problem on the target machine.