Stephane,
I have seen this issue before, I have actually done this before with another customer. Unfortunately it comes down to the fact that there are too many characters in the string. The java has a character limitation that cannot be modified.
There is no fix for this and nothing planned for it. This is such an edge case issue with customers that it will be at the bottom of a bug fix.
Something that you can try is to see what is configured in the Plugin settings. Are you including the group "recepient" in the lookup parameters? You may want to make sure that is unchecked, unless you have some reason that you are looking up some recepient information. This may eliminate the issue.
Just uncheck the box (picture) and then reload the plugin.
Hope this makes sense.
If this solves your questions please marked as solved.
Ronak