In 6.0.2  I encountered this problem as well, its strange issue as I admin you should be able to edit any space. So its bug from confluence side.

In 5.9.7 I encountered this problem as well, mainly on the spaces I did not create. For me the workaround was not to create a new user account, but to just add specific user-access rights on these spaces for my (admin) account. 

Also have the issue in 5.10.
As admin I can edit and save the page. Only when I first add a reaction or inline comment I get the no permission when trying to edit the page.
When editing the page and closing it without saving, I get the blank page. Pressing the back button of the browser gives the no permission message.

I can see how this is occasionally frustrating, but I don't see how this can be a "huge inconvenience". This bug only prevents those with the highest level of admin access in Confluence from editing pages they technically don't have permission to edit, due to either space permissions or page restrictions.

In addition to the other workarounds listed, another easy workaround is to use your admin powers to grant yourself permission first, then you can edit without hitting this issue. I'll add this to the list in the description.

This bug is still open because we would definitely like to fix it. But it is in the queue behind other more impactful bugs right now.

We recently upgraded to Confluence 5.9.10 and are also encountering this issue.

@Matt Ryall- if I understand you correctly, admins should have to use 2 Confluence license seats? One for admin purposes and one for general use? That makes no sense whatsoever. This is a major bug for us not to mention a huge inconvenience for the licensing price we pay. I'm working on admin tasks for most of my day. Does that mean I have to toggle in and out of user accounts while I work in Confluence as well as admin Confluence? Ridiculous!

I agree that this may not be the highest priority issue because people generally shouldn't be using an admin account for regular work. However, I often have to log in using the admin account to diagnose Confluence problems, and then I forget that I'm logged in as that user. When I then start trying to do regular work (i.e. editing a page), I get a nasty surprise when I try to save the page.

So another way to resolve this may be to make it more obvious to a user if they are logged in using an admin account - perhaps by changing colours in a prominent location.

Note that this issue is not limited to pages with restrictions, but also affects unrestricted pages in spaces in which the admin account does not have edit permissions (at the space level).

Luckily I can usually "recover" the lost work by editing the page again, viewing the changes in the draft, logging back in as the normal user, and then manually re-applying the changes. But it's just really frustrating when it happens.

Also, if you don't have permission do edit and click on Close instead of Save, the page gets blank. It doesn't refresh it or it doesn't redirect you back again to the "view" mode.

Same issue over here

The suggested workaround does not work on our site. I removed the membership of confluence-administrators for 'user123' and granted him individual site admin permissions. When this 'user123' now tries to access a space he already gets the not permitted error.

This is not a minor bug for us.
We use Digital Certificates for single-sign-on, so there is no way for users to have dual accounts, one for "normal" and one for "System Admin" work.