Joomla Extensions not uploading

[#1]

I have been getting an error message since earlier last week when I try to upload extensions to any of my joomla sites. It just states that 'There was an error uploading this file to the server.'
I can still add articles, create categories etc, so the basic functionality is there. Images will not upload either.
Anyone else had this problem, or is there an issue with my webspace or cgi?

[#2]

Just guessing, but if Joomla is using /tmp as temporary storage for uploads it is probably full. You could try using a customised php.ini as described in this thread. In particular see Replies #7 and #14.
You probably aren't using sessions so could ignore that part of the thread.

[#3]

Tried that yesterday before I posted, but it made no difference (apart from giving me a sore head as I tried to follow the steps  ;))
Mysteriously everything is back to normal again today. Has SKS been active again?