This could be a tough one. Did you make any recent content changes or customizations that might have led this to happen? Or did the server configuration change? I mean - did it work before? If so, something must have triggered it. Does it happen on all pages? How about just the login page, e.g. tiki-login_scr.php
Clearing the Tiki caches might help - but since you can't login you will have to do it manually. Can you try clearing all the contents of the "templates_c" folder ? Maybe there is something in the Smarty cache that got corrupted. You should also try clearing the contents of the temp/cache folder.
Also Tiki 9 is no longer supported and you should upgrade - but since it's an internal server I suppose it's not that urgent.
While 128 is probably too low a setting, 256 should be sufficient, and since you tried up to 1024 something is wrong.