I am not an expert

but i had this experience to, it has passed 1-2 years and don't remember how I fixed it.
* Have you tried with another browser?
* Have you tried to disable mods than try to send P.M
* Any update made by your server to php - sql version? and it does not meet vb's
When I have issues likes this, I disable 5 mods at a time, it helped me a lot to focus.