Just downloaded GNUcash Portable. I keep my records in a MySQL DB which, on Windows, requires entry of password on every DB connection (i.e., whenever you start GNUcash). The panel used for entering the DB connection parameters does not allow use of keyboard (i.e., CTL+V) to paste data into these fields. Using mouse (right click) to select paste does work.
Once DB connection is established keyboard seems to function normally.
You'd need to let the publisher know. They may have disabled it for security reasons. I personally disagree with that approach as I use a password manager.
Sometimes, the impossible can become possible, if you're awesome!
I can copy (double click) and paste (ctrl-v) from KeePassPortable (which I think most people use) into GnuCashPortable's db login without difficulty.
Wm