Howdy!
Well - I still don't know
WHY? sylpheed (and claws-mail as well) shows this strange behaviour. On the German counterpart to this forum (bsdforen.de) I found one fellow claws user who made the same experience.
After this last incident I had switched back to claws-mail, whch as a result fetched all those >9,000 mails (roughly 1.6G). So here I was with two comparable programs and no idea why both behaved strange every now in a while.
After some sleep and thinking I realized that both (sylpheed and claws-mail) use a folder ~/.sylpheed/uidl/<account-name> or ~/.claws-mail/uidl/<account-name> respecively, to store a list with the mail-ids and a system-id. This brought me to an idea: I took the claws-mail-ids and combined them with the sylpheed-system-id. Et voilá: Sylpheed now "knew" again of the already fetched mails and fetched only those missing!
My workaround will be to set up a little script to save the content of ../uidl/ on a regular schedule. If claws "forgets" his memory the next time I will be able to go back to a date in the near past.
I still hope that s.o. comes up with an explanation of what really went wrong but until then I will have to cope with this issue as described.
Cheers,
SteWo