Hi, I've tried for quite some time now to get Article Sharing to work but either I'm doing something wrong (probably) or there is a bug in the plugin. Some of slaves sites uses this plugin, there it works, if I don't edit/update the menu item, I can't make any new menu items or change article.

I'm using:
Joomla 3.7.0
JMS Multi Sites 1.3.66 BASIC (patches 1.3.62)
Article Sharing 1.3.13

Scenario 1: When I try to make a new menu item in a slave site with "Multisites Article Layout" I can select <Master Site> but all the selected articles are from the slave site.

Scenario 2: If I already have a menu item in a slave site with "Multisites Article Layout" I can't change the article (the box is disabled)

//Christer

When you create a new menu item, to get the appropriate list of articles, you need first to save the menu item with any article and after re-open the menu item to see the appropriate list of articles.

The known issue is that when you select another site ID in the menu item, the refresh of the articles associated does not work. So save and re-open to display the correct list of articles.

Ok, but... when I select article (second time) after I've selected <Master Site> I get: JForm::getInstance could not load file

At which line ?
This is not clear.
Please give the full detail of the error message

When I select a new article normally an lightbox appear with all articles, the error message appears here and nothing else and the lightbox window freezes so I must F5 to come back.

OK but you mentioned "JForm::getInstance could not load file".
So where I can find this message.
Source name, line number, ....

Can you give a screenshot with all the details.