This happened to me also, absolutely no errors reported anywhere…
I had to downgrade back to 2.3.0 and everything started working again.
I´ve been very happy with Piwik for a long time, and been trough many updates without any major problems.
Now I´m afraid we have to go over to the dark side and the galactic federation.
I just posted a message with the same problem (301 Moved Permanently). We think that we have found out that this is because the DeviceDetection module is disabled. When debugging (Tracking HTTP API: API Reference - Matomo Analytics (formerly Piwik Analytics) - Developer Docs - v3) we saw that a coloumn in the table piwik_log_visit did not exist. As it looks, it is created when enable the DeviceDetector module. I can’t confirm this as the db update still is running, but the query looks good. Maybe this helps or, if not, you might find your problem when debugging the tracker response as describe.
Are you serious? For an update I need to activate a beta-plugin which causes a massive failure in piwik?! It’s frightening me.
I will check that after my vacation.
Unfortunately we had a bug in Piwik that requires this plugin to be enabled. Alternatively you can use Piwik 2.5.0-b2 which should not require this plugin to be enabled any more.