[exim] Vary weird problem - unable to set uid=12

This is a very weird problem. Everything will be running fine for
several weeks and all of a sudden I get this:

2009-03-29 06:48:57 1LnvJq-0000sZ-Eu unable to set gid=12 or uid=12
(euid=0): post-delivery tidying
2009-03-29 06:48:57 1LnvD5-0001Jx-7o unable to set gid=12 or uid=12
(euid=0): filter_quicktap router (recipient is andrea@???)

And at the same time I get this:

2009-03-29 04:36:05 daemon: accept process fork failed: Resource
temporarily unavailable

I have some unusual circumstances. I'm using OpenVZ virtualization, and
I'm using the ACL run command a lot in ACLs.

The system has to run for several weeks for it to happen by lately it
does seem to happen regularly after several week. Restarting the virtual
server does not fix the problem. However rebooting the server does fix
the problem, for a few weeks at least.

I know this is probably an OpenVZ question and I am going to ask it over
there. But I'm just trying to understand the problem better before I go
to OpenVZ and ask them. for those who are familiar with OpenVZ
virtualization my VM is wide open, no restariction bean counter show
plenty of resources and no failcounts.

UID 12 is user mail. What would cause Exim, running as root, to stop
being able to switch to user mail? I'm wondering if the process fork
problem might be a result of trying to switch UID as well?

I know it's a weird question. Just trying to understand it a little
better. Thanks in advance.