Basically followed instructions from another thread to upgrade the Joomla from 2.5.8 to 3.6.2

I have a successful upgrade on the master web. Another partially successful on a second web that controls the user accounts, but all the slave webs report the same error:

"Error displaying the error page: Application Instantiation Error: Error initialising the session."

I cannot access the back or front end of these webs

(Structure is

MASTER
Users control
Slave1
Slave2
Slave3 etc.

Master runs with no problems on 3.6.2
Users runs okay, but I cannot use the ISIS template, as it does not load the entire template to manage upgrades, discover or fix database.

I have gone through the Maintenance fixing various DB issues.

I am hoping the "Error displaying the error page: Application Instantiation Error: Error initialising the session." is unrelated to JMS Multisites, but I have tried everything. All configuration.php files are correct.

Is it possible some files need to be manually placed into those directories after a joomla upgrade?

Thanks, anyone, in advance.

In general the procedure to upGrade from J2.5 to J3.6 can be summarized as

- Update ALL the extension under J2.5 and verify that they are compatible with J3.6.
This includes JMS that must be updated before doing the upgrade of Joomla.

- Make a backup to be sure that you can rollback in case of problem.

- Switch the back-end to the Hartho template as this is the only one compatible between J2.5 and J3.6.
BlueStork (J2.5) is replaced by Isis (J3.x)

> Delete (or rename) /installation (that contains the J2.5 one).

- Do the upGrade till J3.4.8 and ensure that all the slave sites are OK.
Don't go directly to J3.6 because J3.4 is the only one that keep the DB structure encoding the same as J2.5.
Starting with J3.5 Joomla changed the encoding of the tables and this may cause compatibility issues with some extensions.


> J3.4.8 - Extension Manager / Install => Upload the Joomla 3.4.8 update package.
> Extension Manager / Database => Fix button
> Go in JMS and re-install the patches.

- Check JMS template that you use in the slave sites and open the JMS template to update the files and folders.
There are new folders added in J3.x
Once the JMS template are updated, you will be able to Open/Save the slave site definition to create the new missing folder.
In case where you have dependencies between sites, verify with the JMS Tool the tables that would exists in a parent sites.
When you will Open/Save the table (and the extension) present in the master will be added in the children.

- Go in each slave and do Extension Manager / Database => Fix button.

If you didn't changed the back-end template into Harthor, you can do it with PHPMyAdmin and table #__template_style.

Once you have all the slave OK then redo a backup

After, you can upGrade to J3.6.0 and after again to J3.6.2

Don't forget to do the Extension Manager / Database => Fix button in each slave to apply the DB structure modification.

If you are using the "user sharing" between websites then you will have to rebuild the sharing.
Drop all the #__users, #__sessions, #__viewslevel VIEWS.
and Open/Save the slave site definition to rebuild the sharing.

We also have a document that explain the migration but it only exists in french
see jms2win.com/en/documentation/migration-joomla