Wow, they actually fixed it!

Managed to report it myself, http://trac.openstreetmap.org/ticket/3479

If somebody has a better idea, you're welcome :)

Yes, they changed something.

Just tried to upload the same file via JOSM w/ DirectUpload plugin. It fails at the very end, too. Wireshark revealed the exact message:

HTTP/1.1 500 Internal Server Error
Date: Mon, 24 Jan 2011 20:54:15 GMT
Server: Apache/2.2.14 (Ubuntu)
X-Powered-By: Phusion Passenger (mod_rails/mod_rack) 3.0.1
Vary: Accept-Language,Accept-Encoding
Content-Language: en
X-Runtime: 1582
Cache-Control: no-cache
Error: Errno::EACCES: Permission denied - /store/rails/gpx/traces/910162.gpx
Set-Cookie: _osm_session=77948e7bd3506c4132aec5a2c13ecb5a; path=/; HttpOnly
Content-Length: 69
Status: 500
Connection: close
Content-Type: text/html; charset=utf-8

It's a shame that such an advanced technique is required to quote the exact message. The error text is in a static widget. There is no debugging log. And nobody will type such a long text from screen, ever. In the Dilbert universe, that would be a typical strategy to make support calls impossible :)

Will somebody report the error to developers? I have no idea who is responsible for this part of OSM. Can't deduce anything useful from osm.wiki/Platform_status .