Search for special char (e.g. Ö) is not working with mkgmap

the following search is working fine:

shop=car_repair & (name~‘Adac.’ | name~'ADAC.’ )

but the following search is not finding any POIs:

shop=car_repair & (name~‘Arbö.’ | name~'ARBÖ.’ | name~‘ÖAMTC.’ | name~'ÖAMTC.’)

I have tried with my editor set to ANSI coding and UTF-8 coding.
UTF-8 coded files gives an Error with mkgmap,
ANSI coded files gives no error but finds no matching.

Is there any way how I can search for “Umlaut” characters?