When I try to update my Joomla Multisite from 3.6.5 to 3.7.0 I get this error: Fatal error: Class 'MultisitesContentRouterRulesLegacy' not found in /home/mettecos/public_html/components/com_multisitescontent/multisites.router.php on line 56

I've bought and installed the latest JMS2WIN version: 1.3.65
I'm currently using PHP-version: 5.6, I've tried to upgrade to 7.0 and 7.1 without any success

Any ideas on what to do?

//Christer

The error is related to the Article Sharing.
Verify that you are using the latest "Article Sharing for JMS" version 1.3.13.

When you upgrade of Joomla, it is always recommended that you use the latest version of all the extensions.

Thanks, that made all the diffenrence BUT... now I get "Fatal error: Call to a member function get() on boolean in /my_path/components/com_multisitescontent/multisites.router.php on line 70"

I get this on all pages that share menu item with master site, do you have any updates for that plugin "laying around"?

I checked again and could it be the "Article sharing" that is the problem (I use version 1.3.13) on both master and slave sites.

Can you also precise the Joomla version that you are using.
Is it still J3.7.0 or 3.7.1 or 3.7.2 ?

I had a look in the J3.7.0 and the line 70 is

$category = JCategories::getInstance($this->getName())->get($id);

That would mean that JCategories::getInstance() return false and therefore, effectivelly the get() function does not exists on a boolean.

The only case where JCategories::getInstance() is when the name of the component "$this->getName()" provided does not provide the code to manage the category.
This can be due when the
JPATH_SITE . '/components/' . $component . '/helpers/category.php';

does NOT exists or when it does not contain the appropriate class name.

To make a diagnosis of the reason, try modifying this line to add test and collect debug info.
Try replacing the line 70 by the following.

$obj = JCategories::getInstance($this->getName());
if ( empty( $obj)) {
error_log( "Invalid category for component:".$this->getName());
return array();
}
$category = $obj->get($id);


Remark:
this peace of code should write an entry into the error_log file and show not return any category.
The code will be removed automatically with the "Purge" menu or any update. So consider it is as temporary debug code.

Thanks, I'll try it out, I'm using 3.7.0

The "fix" you provided solved the problem!!! THANKS