I had this problem for several days and I couldn't access my watchlist AT ALL.  Someone in the forums said to click on My Ebay Summary on the dropdown list that appears when you hover the mouse over My Ebay across the top of the page.  It worked but clicking on any of my other watchlist links didn't work.

It works better in firefox than in IE.

I'd already written to ebay about it before I found out how to access it.  This is their reply:

Thank you for contacting eBay and raising the issue of getting an error everytime you view the watchlist.

We have received similar reports about this from our members and our Technical Team is currently working on it.

I have already added your account information as one of the affected members.

I do apologise for the inconvenience but rest assured this will be dealt with utmost impostance.