Invalid login, please try again after upgrade from 3.5.10 to 3.6.8 - moodle

Question

I have created an identical copy of our production instance to my local environment running XAMPP and I am trying to do the following:
After upgrading Moodle from 3.5.10 to 3.6.8 and restoring all plugins etc. I am unable to login. Before upgrading to 3.6.8, I had already performed the following upgrades,
3.4.1 -> 3.4.9
3.4.9 -> 3.5.10
and everything worked fine during those 2 upgrades. I followed exactly the recommended upgrading method as described in moodle's docs.
After successfully upgrading to 3.6.8, I successfully passed the step about the server requirements for the correct php mysql and db versions, I then successfully passed the next step as well for upgrading the database and made sure that all the plugins are fine and had no errors (just some warnings about a few plugins that can be updated to newer version) and after that step finished too, I then got back to the /admin/index.php screen asking me to login. That seemed weird to me because during the previous updates, it never asked me to login after completing the upgrading, but I said, ok. i'll just login. So after trying to login i am constantly getting the message "Invalid login, please try again"
I have tried almost anything here and by anything I mean the following.
First of all, I checked my hashed password in the local database and it is exactly the same with my hashed password in production database.
I then read an article that from moodle 3.1 the logintoken was introduced and since we use the mb2nl theme, I supposed that I just have to add the new input hidden field for this logintoken to the login form but the field was already there and again that was not the issue.
I am at a point where I cannot think of what else might be wrong and I am stuck.
A few more information about our whole setup, we have a Joomla website and our moodle users are actually coming from Joomla, Therefore we use a plugin named Joomdle which connects these 2 CMSs. Our users register to our Joomla website and the Joomdle plugin sends the credentials to moodle and the user is being created to the moodle CMS as well and an entry is being created in the moodle's "user" table. The difference is that moodle generates a different hashed password from the hashed password that joomla does. Anyway, there were no problems at all with logins or anything else since I tried logging in with multiple user accounts in moodle and all worked fine, before upgrading to version 3.6.8.
This is not a Joomdle issue since I already checked this by completely removing Joomdle and this is not the cause. This is probably coming from moodle core but since moodle has NO error reporting or logging at all, I really cannot understand what this is.
To exclude one more matter linked in this question, I already checked and made sure that moodle's password salt is the same both in production and local environments.
I am not sure what is going on here and this is why I desperately need some help from experts.
Solution:
I was able to solve this problem but forgot to come back here and give the solution.
The problem was that, as I said above, we are using the mb2nl theme and the theme's login form did not include the hidden input field to create the logintoken.
On my question above I write somewhere in the middle "I supposed that I just have to add the new input hidden field for this logintoken to the login form but the field was already there and again that was not the issue". I was looking at the wrong place. The login form that I was looking at, was the Moodle's itself login form and not the theme's login form.
I then found the login form in the theme's files and the logintoken hidden input field was indeed missing from there. I added it and then I was able to login normally.

Answer 1

Happened to me today. Executing the cron.php cli script trigered again the upgrade process and solved my problem.