Notes Client crash 'abnormal program termination' when opening mail

this is a mail from one specific sender. I can copy the mail doc to another database, but every time I want to open or forward the mail, the client crashes saying

Microsoft Visual C++ Runtime Library

Runtime Error!

Program: …\NLNOTES.EXE

abnormal program termination

I can however open that same mail using Eudora against the Domino Server. The mail is signed and the certificate is attached as smime file.

Also, same with 6.5.4 (int’ english)

Any help appreciated.

Stefan