Issue with SSI login

[maxwl]

Sorry if this question is already asked, I did a search for SSI but did not find a related question.

I have a local forum setup with the following: SMF 2.1, Maria DB 10, PHP 7.0. (Linux Mint 18)
The forum is configured to use global cookies.
I have integrated the forum with a website running on the same domain. Website is located at /var/www/html/site and the forum is installed in a subdirectory at /var/www/html/site/forum.

I am using the SSI.php located in the root of the forum folder for the integration. Problem is, I can login normally on the website, but if I try logging on through the website I am getting this error: Your session timed out while posting. Please go back and try again.

The log files are showing the following session_regenerate_id(): Session write failed. ID: user (path: /var/lib/php/sessions)
Write permission is correctly setup at /var/lib/php/sessions because when I login from the forum itself, a file is created in the folder. There is only a problem when using SSI.

I have been battling this issue for the past two days without resolve and is wondering if anyone can give some suggestions how to solve.

[Kindred]

what are your other settings for cookies and sessions?

[maxwl]

Here are the settings for the cookies and sessions
Cookie Name: SMFCookie735
Default login cookies length:  60  minutes
Enable local storage of cookies: off
Use subdomain independent cookies: on
Main domain used for subdomain independent cookies: blank
Force cookies to be secure: Disabled
Force cookies to be made accessible only through the HTTP protocol: On

Use database driven sessions: On
Allow browsers to go back to cached pages: On
Seconds before an unused session timeout: 2880 seconds

Two-Factor Authentication: Disabled

I also have a valid database username and password set for the SSI mode

[albertlast]

Which version of smf 2.1 you use? (source / Beta?)
Similiar issue are fixed in the github version some time ago.