I just updated to 5.1 and when I try to save a document I get the message "Couldn't auto-save document" how do I fix this problem?

I have this problem, too. I received a document that I wanted to save, and I got the "couldn't auto-save document" error message when I tried to save it that way I normally save stuff (Click File, then click Save). Thinking it may be some sort of non-compatible file, I tried to save by clicking "Export to...", and still got the same error message. I finally clicked the RED/Close button at top left, and it let me name & save the document.

This is highly inconvenient, and not a true solution, because it only let me save it under Documents. I had to go the extra steps to drag the file to where I actually want to save it. I don't know what the problem is, and I wish someone would "fix" it, but I did finally get to Save my document.

A lot of people are reporting this. I haven't encountered it myself.

Most seem to be related to problems with iCloud, either saving to iCloud or having copies on iCloud.

Possibly you could take this to the iCloud forum.

The problem does not seem to occur with Pages '09 that I can tell.

Personally I believe this is just another of the vagaries of working with iCloud, something best avoided, certainly not to be relied upon.

Peter