I have been aware of this issue for a while now, creating an issue to keep track of findings. Very occasionally, effect entry stops working in the pattern editor. Switching to another module type typically solves the issue because the shortcut list is regenerated then. As this issue happens very rarely with no clear steps to reproduce so far, it's difficult to debug. I've seen it a handful of times and heard similar reports from other people.

Random theory, but seems unlikely: OpenMPT gets a bogus keyboard layout change notification, recreates the key map but is unable to map letters to virtual key codes. I only have US and DE keyboard layouts installed in Windows so this really shouldn't be happening.