After several weeks (or was it even months) of being dead, OSDN just “magically” was resurrected. At least the web page now works again, albeit rather slowly, (but it never was fast to start with). Somebody must have woken up there.
I’ll keep moving my projects back to SourceForge anyway (nearly finshed that already). I can’t trust anybody who let the page be dead for such a long time without even posting a notice somewhere or reacting to emails.