try using a single line of text for your variable and update your person field with that

Originally it was a single line of text type variable with the same error so I converted it to a people or group type.

ui: I dont want to update the people and group field, I use it only to match the correct line in the list.

OK how is the variable populated?

I have set variable action where I assign to KJOHR (people and group type) a single line of text variable value varOHR.

Sorry we use Hungarian translation of Nintex 

Next step use KJOHR at Update Item action as "where" Résztvevő OHR = KJOHR set column value to "Mentve"

The WF stops at this point so I guess it cant match the two values. As you can see in my first post they are identical for value and type as well.

Can you set the property of the People field to Name/email etc. using the below highlighted button and try?

I tried a lot of option from there, but none of them works.

I fill like I am an expert in findig Nintex limitations and "bugs".

How is the value assigned to OHR? Hope there is no whitespace?

It is coming from a collection variable that is processed and every OHR saved as single line of thext (varOHR). First I thought the problem is that it have different type of data thats why it cannot match, but then I assigned it to a person or group variable (KJOHR) with the same result (error).

I was told by a developer that the database my add some more information to the people and group field in the list which I cant see and Nintex not showing. This way it cant match my variable, even if its the same people and group type value.

In my list I need to add one more column where I copy the IDnumber of the users in a single line of text field, than it will match. 

Did this work?

I didn't have time working on this since, but it will work for sure.

I don't think it's a matter of adding and hiding anything.

if you make a query to sharepoint list and want to filter by people and group field you have to do so by Sharepoint's user ID.

here is some example, even though not directly nintex one, but I believe it might give you a good hint Using CAML to compare SharePoint "Person or Group" fields - Stack Overflow 

so within nintex workflow, get SP's user ID (eg.) ith with set variable action and respective return type and then configure update item action to filter by that user ID.