There is issue in version 22.214.171.124 with user cookie/session being broken. Whenever I load Splunk, I do receive 404 on this address:
As you can see used == undefined. After some time this issue goes away and returns back.
Temporary solution: Just run Splunk in anonymous mode.
Unfortunately, I didn't found a way how to reproduce it but I hope Splunk will fix that in new versions.