I was editing a collaborative wiki with forced HTML format. One of the pages was giving me trouble so I decided to use Creole instead. I had to change the wiki settings to remove the forced format, Then tried to delete the relevant page in order to re-create it. As soon as I deleted it I got the Invalid course ID error as described in this issue: MDL-51198

As in the case above, all wikis became inaccessible. Doing a full refresh to clear browser cache had no effect but I found it was OK in an incognito window so I logged out and logged in again on the original browser and that cleared the problem.