Thunderbird HTML mode Linux

I am Using Thunderbird on openSUSE Linux.
LT is only working in text mode, not in HTML mode when answering to a mail.

There is the blue sign that all is OK, even when typing wrong text.
Starting answering with the Shift key (text mode) LT is working as expected.

LT is working when starting a new mail.

Do you use a signature for your emails? Do you maybe accidentally type into the signature area? If that’s not the case, could you maybe post a screenshot?