Sessions issue for some customers? Blank page.

Hi (i'm running 1.3.9h)
Some of my customers are having issues accessing the shop. Some have tried a different browser, but are still only getting a blank white page when trying to access the site.

I've tried myself on different computers and with different proxies but cannot recreate the issue. I've contacted my server host and registrar, but none have any suggestions.

The only thing I can think of is that I ran an "optimize database" from within Store Manager like two weeks ago and that it perhaps caused a session <--> cookie issue. But I'm no sql junkie and don't quite understand how it actually works :)

I did a DB backup prior to the optimization though, but am afraid to restore it just like that.

Can I... perhaps empty the sessions table to "reset" the ids' or how does it work? Restore only the zen_sessions?

I've looked into /cache for debug logs, but the last one was created a month ago. I also have full logging enabled on my server, but it's kind of difficult to sort out which lines actually are interesting..

Nevermind that, I figured out how sessions and cookies work and got an update from the host that told me the issue is with another server, and that it only affects a group of visitors behind a certain router. The router will be fixed "soon"