This is an esoteric issue I realise however after upgrading a site to 3.9 latest the activity picker in a course would hang on the spinning circle.  The issue was reported in the browser console as an issue with a non-existent user.

The underlying problem was that, in this site, the guest user (id 1) no longer existed in the mdl_user table.  The code in lib/accesslib.php line 6357 calls a local context which does a check against the guest user (id=1, deleted = 0 and MUST_EXIST) which subsequently fails causing nothing to be sent back from the ajax call.

I was able to fix the data by adding back in the guest account (no Idea why there wasn't one but the site has been working fine since at least Moodle 3.0 without it.  Possible fix would be to check that there is a guest account and re-create it if it's gone missing, making sure that other sites which may have this same broken data don't stop working for users.