I had a 60 day trial of DO 12 Pro running and tried to access my smartphone connected via USB cable. Clicking on the phone in the drives view switched the lister to the FTP sites instead. No error dialog. Now with my full license MTP works again.
This is a bug. Even if the 60 day trial doesn't support MTP, it shouldn't go to FTP sites.
Other than the expiry, there should not be any difference between a trial version of Pro and Pro itself. MTP is supported in the Pro trial version.
Has anything about the phone changed at the same time, e.g. an OS update on the phone? When the problem was happening, did you try doing the same thing in Explorer to see how it reacted?
I changed nothing between the trial and full version. Same phone, same OS, same USB hub I use. The phone worked in the Windows Explorer at the time and it does now. My trial was for the Pro version.
There's no code in Opus to switch from MTP to FTP automatically.
I'll try to replicate this on a second computer when I have time.