I have the same problem (in my case, the button with the ! does not work (on a swiss german keyboard)).
I tried to detect the problem with AutoHotKey (I could not see the button or who influences it neither) and KeyTweak (did also not work).
The current workaround: set up a hot key via AutoHotKey that inserts the button ! via the clipboard (the Send function of AutoHotKey also does not work). Save the following in the file exclamationMark.ahk and run the script via AutoHotKey:
; workaround to make "!" available via AltGr-5
; Save the entire clipboard to a variable of your choice.
ClipSaved := ClipboardAll
;Send ^v ; does not work in cygwin mintty terminal
Clipboard := ClipSaved ; Restore the original clipboard. Note the use of Clipboard (not ClipboardAll).
ClipSaved = ; Free the memory in case the clipboard was very large.