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I got this token verification error when changing back to default theme via admin cp. Log out and re-login but the result is still the same.

1.1.

Don't bother. I have deleted the upgraded site and its database. I don't think the upgrade works, fully.

Usually token verification errors are completely unrelated to potential upgrade problems because tokens were present in 1.0 as well and they do not use the database (that is the only difference between a fresh install and an upgrade).
So, if it is a token issue it's going to be broken in a fresh install as well, while if the upgrade has an issue it's good to understand what the issue is and try to fix it, so a general description of what could be broken would be useful.

Unfortunately, the successfully (though not fully) upgraded site and database was already deleted. The token verification is not only affecting changing theme but also the other settings as well. In short, I simply cannot save any settings due to it.

I have tried to restore and upgrade the 107 site but the upgrade now stuck at old language error. I couldn't get it to pass that, so that was it for the upgrade.

Anyway, the install of 1.1 Beta 1 or 107 or the restore of 107 site is running just fine, so I think the issue is only related to upgrade. If I can go through with the upgrade, like I did the first time, may be, just may be, I can check the error again.

Doing some digging and found this was once an issue in SMF 2.1 Alpha. @Burke Knight was reporting this and the issue was solved by Oldiesman. I am not so sure how to check it further since there is no link to that fix. Could this be related to this? I am not so sure until I can upgrade again.

The two things are most likely fairly different.
First of all it is important to know exactly what were you doing.
It's coming from an upgrade, so I guess you had a custom theme and wanted to go back to elk default.
I guess: admin > config > theme management > manage and install?
Then were you changing the "Overall forum default theme"?

When the upgrade was successful, I did change the theme to FaceIt to test how far it is working with 1.1 beta 1. Everything seemed normal except for some like the font awesome for menu is no longer working.

Then, I tried installing ROAM addon for rearrangement of the admin menu. It works partially. So I removed the hook using the tool since I cannot uninstall it.

Then the token verification error came. Since I couldn't save some of my preferred settings, what I did was, I changed my theme to defauly using profile page. That works but the token verification error was still there.

I think I can just redo the upgrade (since I have all the backup except that upgraded one), so I deleted the site and its database.

Unfortunately, this time round I cannot upgrade with old and new package even I am using the same files and database. All that I got is old language error.

So that was the full story but I really not sure what was causing that token verification error.

Same error here on a 2.1 imported site.   Except in reverse.  The token error occurs on an added theme but not the default Elk.  The other theme is set as the forum default, if it helps. 

So you are changing the theme in the profile?

Setting it back to the Elk default in a user's profile gets rid of the token verification issue, if that helps.  Please forgive my poor explanations. 

Don't worry, I'm not in a good state of mind (headache due to some insect stinging me in the head and sleep deprivation are not a good combination LOL).
I mean the error token is always in the profile page, not in the admin one, right?
Because in the original report I had the impression it was in the admin panel, then at a certain point I understood it was in the profile page, so now I'm a little confused...

For me it happens in the admin panel or profile whenever attempting to make updates. 

Could you try to change the theme here?
Just to have a verification.
I added a quick&dirty color replacement for testing.

Changed theme, did a profile edit.   Both were successful. 

Which addons do you have installed (specifically do you have the fancybox addon ?)