I also was using nightly svn version with no problems. But when I tried to install rc1 in two Debian Sarge servers, I have exactly the same problem. One of them uses Apache 2.0.54 and PHP 5 and the other one, Apache 2.0.54 and PHP 4. After applying your proposed changes, Roundcube backs to work like a charm.
However, for some reason, in a Debian Etch server (Apache 2.2.3 and PHP 5), everything works fine with rc1. No changes were needed. So, the problem seems not to be in the client. Would it be some Apache related problem?
Best regards,
Robson