Strange fontconfig problem

Hi all, I've got a strange problem with fontconfig on CentOS 7.6: with fontconfig version 2.10 the font display of an proprietary Linux software (based on python-2.6 and tkInter) is fine. After updating to fontconfig-2.13 the underlines used e.g. for key accelerators of tkInter buttons or menus are displayed as black boxes (see attached picture). Does anybody has an idea how to fix this? The solution might be something in the package configuration but I have no idea where to search for this.

Best regards, ChrisF.