Hi, mariuszmp
I was able to identify the reasons behind the issues after debugging. It turned out that there were conflicts with the JCE Editor browser in the image fields. I've managed to find a solution for the thumbnail image issue that occurred after choosing and saving and updated the code on your site.
The file that has been modified is: plugins/system/jacontenttype/layouts/joomlart/fields/items.php. I made changes to lines 120 to 157.
Please take a look at the changes and let me know if you need any further assistance.
Regards,
Vanhs