I notice recently that in Syncrify when I try to logon via the web logon (when run on my Mac - in either Chrome or Safari browsers), when my server is busy running backups, I cannot logon to the console using any user as I get Invalid UserID or password continually. I have 2 users, my email address and Admin - both get the same error. If I try the same logon on my backup server using IE11 to http://localhost:5800/app, It allows a logon every time to both users. To resolve the issue I need to rerstart the Syncrify server through the web console on the backup server, and then I can logon from my Mac perfectly again.
Syncrify - Version: 4.9 - build 1027
I busy server should not cause this. I have a feeling that your Syncrify server has blacklisted one or more IP addresses, which happens when invalid user id/password combinations are submitted from one IP. This problem gets compounded if every client appears to connect from the same IP address.
I'd recommend you refer to Syncrify.log file to confirm if your IP is blocked. This file is located in $INSTALL_DIR\logs folder.