Using Mp3Tag 3.23 / Windows
In 'Tag Source Configuration', just opening the window (without changing any fields) and exiting by clicking 'Cancel', the "close window" icon, or pressing 'Esc' works fine. However, opening the window, filling/selecting/enabling any field and exiting only with 'Esc' (without submitting) will result in the following dialog:
Closing this window using 'Esc' doesn't work; closing via 'Cancel' / close icon and reopening returns the window to normal, but the issue can be replicated following the same steps.