Hmm, I haven't seen this one before. Are you able to reproduce the error or did it completely go away on restart? If so, the next time it happens, could you send me the log files (Player.Log and Player-Prev.Log). This will show any errors happening under the hood.
Those files are found here:
Location on Windows: C:\users\you\appdata\locallow\Unknown Origin Games\OddRealm
Location on Mac: ~Library/Application Support/com.Unknown-Origin-Games.OddRealm
Location on Linux: ~/.config/unity3d/Unknown Origin Games/OddRealm
I'll do some tests with the window to see if I can reproduce this on my side.