The solution on this problem is to change line 213 in index.php from:
header('Cache-Control: public, max-age=300');
to:
header('Cache-Control: private, no-cache="set-cookie"');

I'm not convinced. With the code the browsers should cache the pages (for 300 seconds (5 minutes)). The IE was excluded from this solution because of problems as stated in the comment in front of the line #213 (#211 f.).

What the web browser SHOULD DO and what the web browser ACTUALLY DOES is two complete different things.

By doing this change, and this change only, the problem was solved.

Unless the web server is very busy, there is not much need for cache. For the web sites I'm responsible for, web cache is not needed at all.

Best regards,
Daniel