I don't think there is a real solution yet, but to try to further pinpoint the problem, you could try the following stuff:
First thing to do would be to test with another contact, so maybe you can add someone else you can test with?
If it still occurs, uninstall Messenger Plus! Live to see if it is realy a problem of Plus! itself.
But, before you uninstall, export your all your Plus! settings first, unless you didn't alter the default options of course. This in case you decide to re-install/downgrade back to Messenger Plus! Live 4 after doing all the stuff below. Otherwise all your Messenger Plus! Live 4 settings will get lost.
If it doesn't occur anymore after uninstalling Messenger Plus! Live 4, it is almost for sure a Messenger Plus! issue.
So, in that case I would suggest to update to the
newest version of Messenger Plus! (5.01.706) and check if you still have the same problem.
Note, when you update to Messenger Plus! 5, all your settings of Messenger Plus! Live 4 will be automatically ported.
If it still happens with Messenger Plus! 5, then I would try to disable the following two options:
Plus! > Preferences & Options > Appearance > Chat Window > Increase max size of messages you can send
Plus! > Preferences & Options > Formatting > Formatting Options > Improve pasting operations for large text
After that, restart Messenger and check again if you get the same strange stuff.
And finally, it might be a very very long shot, but you could try to disable BitDefender (just to check).
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