Before upgrading to 7.2, I had two users saying they couldn't initiate a PM (although they could reply). Upgrading to 7.2 fixed the issue for them.

I have another using saying he still can't use the "Preview Post" button. He's on a PC on a corporate network, with most likely a firewall.

Wondering if anything might change in the JavaScript for buttons like "Preview Post" in 7.3?

Can he post at all? or just not preview?

It could be a timing issue, a browser, Antivirus

You would need to get more information from the user to determine the actual cause.

When i went 7.1 to 7.2
members said that the switch to full reply wouldn't work

After CTRL + F5 a few times they reported it fixed

I assume you have asked them to clear there temp internet files?

yeh I made rick redo the pm system; as it became more and more popular of a problem for more and more users (other than me!)

Some systems security settings (or browser settings) may b0g ajax (javascript in this case) so it'd likely be that culprit