Thanks a lot for your hints.
In meantime I found out, that the problem seems not to be a PLONE problem, indeed. But still it is strange:
The Problem occures in some of our PLONE Sites, but it is no Problem in other ones of the PLONE Sites, that we have. It is independent of the PLONE Version (we have 3,4,5).
We use Apache and the error logs, were the Problem occurs says on every upload try with files >500kb:
errorThe timeout specified has expired: proxy: prefetch request body failed to 127.0.0.1:8099 (127.0.0.1) from 126.96.36.199 ()
And sometimes, but not always:
[error] proxy: pass request body failed to 127.0.0.1:8099 (127.0.0.1) from 188.8.131.52 ()
Andreas, the strage thing is, that the problem always occurs, when I call a site by its portnumer instead of its domain name, thus bypassing the Apache server, that we use. If I call the same site with its domain name, it works fine (as said for some of our sites, not for all of them).
We have now asked our provider, who hosts our server, to check the thing. But, maybe - even if it seems not to be a PLONE problem - some of you might have an idea about that.
Thanks for any help.