Just in case this information proves useful to the ThinLinc developers or other users, I have found by chance a workaround to the problem described in the subject line: “Stuck on the splash screen or a black screen during login”
This problem started appearing randomly after I installed ThinLinc server version 4.17 on Ubuntu Desktop 24.04. I have been updating Ubuntu, but not the ThinLinc server. I have used clients for both Windows and Linux, on both Intel/AMD x64 and ARM architectures.
After many failed attempts to link the issue to some aspect of the server or client configuration, I have concluded that the problem can occur… if the client mouse is moved while logging in. Simply not touching the mouse seems to ensure a 100% success rate for logins.
If a login session has already become “stuck” due to this issue, next time you must select “end existing session” for the trick to work.
I would be curious to know if anyone else has encountered this situation and discovered the same “trick.”