There are three files that are specific to a contact: The transscript, contacts and the VCARD. Both are stored in the user profile. This is located somewhere (depending on OS and language) . . . \Spark\user\email@example.com
To fix the issue, I would delete the content of:
If this does not help, please delete th econtent of
IMHO that should fix the problem.
The last option (more work) is to delete the transscript for the contact that can’t be opened. These are located at
Let us know if that helped.