I'm using 18.04LTS/Gnome.
I can't re-enter my reg. code anymore because it was, somehow, accepted when I pasted it in the first time, even though the example reg.code did not disappear from the screen at the time I hit the enter key. I also noted that the "Registration Info" screen did not show I was registered at that point. It was only after I tried this several times, that I made the post figuring it might be a bug. The following morning, I received an email with a new "mjr" file from JRiver (it must have come automatically when I pasted in my Reg. Code the night before). I never used the "mjr" file because when I restarted MC25.0.15 and looked at the Registration Info box, I saw that I was now registered. As I said, it's a mystery to me what happened.
I think this is/was possibly a one time glitch after I installed the new MC25 from the command line (as opposed to the "deb" file); temporarily keeping MC24 in parallel. I have since removed MC24 from my computer and MC25 seems to be working OK.