where exactly does this error appear?

can you post webpart configuration?

do you have configured linsk to/from other webparts on the page?

The error appears on the page where I'm trying to add the Nintex Form Web part.

Find below the web part setting:

I have not configured any links on the web part.

Note that, I'm using the same configurations but I changed Form Mode to Edit & it is working fine.

all the resources I've seen reports that it means a problem in content type specification.

I even was able to reproduce your scenario if I used non existing content type.

so can you make sure

- Item content type is assigned to referenced list (doesn't it have eg. a localized name?)

- you typed in content type name correctly (eg. if you copy&pasted it, make sure there a are no space characters around)

- try to supply content type ID instead of name

- since you use right to left direction try to type in content type name in reverse order (just a blind guess )

You were right. Issue was in wrong content type. I was using site with arabic regional settings therefore, the content type was translated. 

Thank you very much!