I just got this error while logging in to my board on my phone (using the PWA). It still logged me in but I'm not sure what this error message means or why I'm getting it? (I'm on the IPS Cloud)

As mentioned in the message, it tends to be caused by theme or plugin issues. Are you just on the main page there? Was this something that happened the once, or are you able to replicate it?

Just on the login page. It's happened a couple of times before on my Mac, but it does not happen on a regular basis.

I would start by testing on a default theme first of all. If thats not the cause, then go for apps/plugins

I sometimes get the CSRF error when I am logging in between accounts. Refresh the/go to another page and it's gone.