Help/More Troubleshooting, Profile Folder, Open Folder, close TB, delete session.json. Restart TB a few times and see if the Message Pane setting is retained.

does not help, it always opens with the reading pane open

Try this: open the profile folder, close TB, delete xulstore.json and session.json, again. Restart TB and test the pane setting.

Did that and TB still opens with the message pane open, this issue started a couple of updates ago and will not go away. I have to press F8 every start to close the message pane

Unless you have an app like CCleaner that resets TB settings, test for a flaw in your profile by trying with a new profile: Help/More Troubleshooting, about:profiles, create a new profile, add an account, and see if the Message Pane setting is retained. If it is, local mail, address books etc. can be transferred from the old profile to the new.

I do not have CCleane and it happens on both my laptop and my desktop. I have no idea why, everything works fine except that TB always opens with the reading panel active.

What add-ons have you installed?

I have OWL for Microsoft Exchange for my email is the only add-on

I'm also using the Owl add-on, and I'm seeing the same thing. I'm not seeing this with any other ordinary IMAP account. So my best guess is that it's related to Owl. Did you contact the add-on author about this yet? That's what I'd suggest anyway.

I will email them and see if they can fix it then. Thanks for the reply