Problem opening PDF-Attachments in sandboxed Mozilla Thunderbird

I need your help!

From one day to another sandboxes Thunderbird stopped opening PDF-Attachments.

Running Thunderbird unboxed, PDFs opens without Problems with Foxit PDF-Reader.

Sandboxie V5.3

Thunderbird V68.6.0 32 bit

Hopefully someone can help us!

Thanx

Alex