Are users already logged in and they are trying to log in again? Maybe add something on the page that skips the log in page if they are already logged in. Maybe that is the issue?
Also, for testing purposes, I would disable this workflow and make a very simple login workflow and test that to pinpoint which step is making it break. Add one step back at a time until it breaks. Then you know which one is causing the issue.
I would also recommend sending what you can to a backend workflow if possible, so if it breaks, it doesn’t interrupt the users flow. Like the send an event to mix panel step.
My first test would be to just have step 1 -login, step 2 - go to page. See if that works properly.
It very well could be an issue with one of your api calls. Maybe an API did an update. That could break a workflow. Once you have isolated which step causes the issue, then you can contact the API support and see if anything has changed or something.
You can keep adding each step back one by one until the error happens again to isolate it. It might show up in a step-by-step potentially too. Either way should work.
If you put these workflows on the backend, this will help to not have these issues in the future. The users won’t be affected if the call is done on the backend workflow.