keys on keyboard return different characters

[Tiemo Hollmann TB]

Hi, have you ever experienced the following phenomenon?

A customer of me tells me that pressing a Umlaut key (äüö) on a German
keyboard sometimes shows the correct Umlaut in my LiveCode program, but
sometimes only hieroglyphs. Sometimes the correct char shows up after having
hit multiple times / or harder the same key. I am using the old standard
font Tahoma on stack level, inherited to all fields, which obviously is
installed on the machine, otherwise the correct chars never would appear.
Since this phenomenon only happens in my LiveCode (8.1.4) program, not in
Word or any other program, it must be related to LiveCode. But how and what?
Keyboard driver or foreign system font or language would affect all programs
on this machine.

Ever experienced anything similar?

Tiemo