I haven't touched the .htaccess file at all, and I haven't moved the installation folder.
Well, Osclass or any other software *usually* don't break without a reason, so if it was working before, and now it's not working, something has changed.
.htaccess seems fine to me, did you modify the configuration of permalinks?
You could delete the .htaccess and access your admin panel, if fact, you could access your admin panel even if the .htaccess is still there, it should cause any problem, but to make sure, you could delete it,or just rename it to backup.htaccess ... access your admin panel and check your permalinks configuration, or disable them. If the problem continues, then it's something with your server configuration. Later, reenable permalinks and check if the problem continues