See bug report in http://forum.piwik.org/read.php?2,78043,page=1#msg-78088
the bug is that, now that assets are loaded from libs/jqplot/, if the directory doesn't have the same read permissions, the assets could fail if some files don't have right permission.
It would be nice that the assets generation would show the error message. Users have reported error 500 without message instead.
Like a user said, jqplot is a beast. It is mate. Not much of a quick fix I don't think ;)
The asset generation is via proxy. We can't provide an error message directly. We could change it so the asset manager returns the error in its output (i.e., return a script that opens a popup).
Low memory webhosts are going to have a bigger problem with archive processing, and in either case, isn't trappable. Defer this aspect for mitigation by #2514.
OK I guess, if we get this error often, we can create a FAQ :)