by Matteo Scaramuccia.
Hi Nic,
you need to contact your Hosting Provider, since the error means that you've an hard limit in the number of files being opened at one time: they should check both the number of files being opened under your account and the limits of your account.
It could be also a bug in Moodle if Moodle doesn't correctly close some of the file handles already opened but first you need to ask if you have limits in the number of files being opened and if that limit could be reasonably increased.
HTH,
Matteo