CASproc.exe (V2.6) instances can lock up while receiving emails is resolved as a bug.

When Outlook is getting email, CAS spawns numerous CASproc.exe processes in the framework of Outlook. These are usually unloaded when send accept completes. Occasionally Outlook fails to download the email, and the CASproc.exe processes appear to remain after send receive completes, even if send receive is not completed. It may be that Outlook fails to send receive because the casproc.exe are locked - certainly the usual Outlook 'cannot contact server mail are not received. The problem then seems to become swelling, with more CASproc.exe processes locking on each send receive. Fully closing and opening Outlook seems to solve it.
Uninstall CAS & CSE & all is well - excluding occasional usual 'server timeout' messages. Disable CAS & all is well. Re-enable CAS trouble starts again roughly immediately. So it seems CAS (or possibly CSE) specific - will update if any know-how to the contrary.

Each in receipt of on one account will generate one CASProc.exe process. This process will do some background jobs for Challenge/Response policy.
For POP only supports one session for one account at a time, and CASProc.exe will hold one conference for downloading emails from distant servers, mail clients may fail to set up one new conference for the same account, that is the reason you are getting 'server timeout message. This happens when you doing in receipt of frequently.

I can see that the email client may not set up new CASproc instances, if the earlier ones (communicating with the identical account) have not finished. I guess this depends on the association between the server timeout set in the email client, and the occurrence of download. occurrence of download is high in my case.