Does the mail contain an Image file? With Version 7.8 there was fixed Problem P5996, that exactly describes your problem:
P5996: "The path is not of a legal form" on inbound emails with embedded PNG image
Which Version do you use?
This has been logged as Problem 6248 .
Please log this issue with your local support provider and reference this discussion.
The customer is on 7.8.2.
Okay, it seems, as Paul wrote, the problem found a way back in the product...
is it fixed with 7.8.3?
This error is still seen in 7.8.3.
Please log this case with your support provider.