Thank you for providing the link to the feature service @AnaGrdan, I am able to reproduce the error when working with the service. I see the service is using a local projection and it's possible this is contributing to the issue. For this particular projection, there is not a transformation available to convert to Web Mercator, which is what the sites use. Additionally, if I use my own Croatia Counties, projected in Web Mercator (102100) with your variables joined, then I am able to avoid the error. I can help confirm this if you would like to provide access to the source data for me. You can just enable the Export Data option on your service in ArcGIS Online from the Settings tab, or you can send along a fgdb of the data. Looking forward to hearing back from you.