Many thanks to Nick T. for reporting that Google™ broke uploading (saving) of Google Bookmarks, probably yesterday. The issue is now fixed - download the latest version (4.2.4) and the "Error 400 Bad Request" should no more be seen.
It was initially upsetting, but I'm glad they did it this week, instead of next week, when I want to spend as much time as possible at the Apple Worldwide Developer Conference (WWDC). It's going to be a long week, taking CalTrain from Santa Clara to San Francisco every day. But not as bad as having to stay in a hotel!
Now I have to go and issue Nick that free license I promised him for submitting such a quick bug report...