Actually, I already troubleshot this issue and got the answer, but I posted my issue as a question so others who face the same issue can find it here.
The main issue is power automate automatically trying to go into the new editor even for old flows. As some old flows are (probably) incompatible with the new editor, they will crash into this 403 page.
The solution is to edit the url after clicking 'edit flow' or 'edit', depending on which page you are in.
Change the url from:
to this:
Power automate will then load directly into the old editor and your flow should be accessible.
