Depending on how helpful your host is, they may be able to point you in the right direction.
Gut reaction is that it’s a clash between plugins – the usual solution for that is to remove the likely offenders and then reinstall one by one.
It could also be as simple as a cookie error – try logging in on a different browser or via a private/incognito window.
Not totally recommended but you can usually “remove” a plugin by deleting it via your control panel or FTP access. Be careful if you do that.
Fiverr can also sometimes be a good place for solving this kind of issue – explain what needs to be done and initially contact 2 or 3 possible vendors rather than buy the gig without talking first.
