can't track multiple users at same time in web

Open two browser tabs on different users.

Click track in both tabs

Put your password in in both the popups

Click continue in both

One usually works, the other fails with the error:

failed to post to the server: bad signature: Wrong previous hash; wanted 'x' but got "y"

Reproduce the issue by opening two browser tabs for different users and starting tracking in both, following the password and continue steps. Inspect the server response and the reported previous-hash mismatch; done means both tracking operations complete successfully without the bad-signature error.

Written by the indexing model from the issue text.