Umlauts not working correctly

If i use the wp-mailto-link to protect emails with umlauts (vowels with dots) each letter is replaced by two question marks.

The click on the mail address works correctly and also the address appears correctly in the email software.

Version 2.1.7

Seems like there was the same issue with the old plugin https://www.ads-software.com/support/topic/german-umlauts-not-working/