Yahoo mail shows part of the web page when using "compose"

This is happenning at a place where we have 11 computers and 1 as a
server, we sell internet. In our country, that's called a cybercafe. I
don't know if you guys use the same name...
Anyway, when using compose mail, I only get a fraction of the text.
This happen randomly on all the PC, sometimes it happens, sometimes it
doesn't.

We tried disabling the porn blocking software and it still happened. We
didn't uninstall it though.

Any ideas on what could be doing this?

Thanks!