Random data entries are not saved

I have very odd problem with my survey. It is open survey (but not listed on the front page). Users got the link via email. No tokens. Their job is to transfer data from hard copy questionnaires to the e-form.

20 users x 10 hard copies. There are two requested questions (the first and second one) and about 60 non-requested.
After they finished with one entry they got Success message and link to another copy. But some of their data entries are empty even they got no error and there was confirmation message that everything is OK.

There are two cases: random entry is completely empty or there are answers only to the requested questions. I did not notice any pattern which of them would be like that.

The data are not saved in the sql database either.

Cookies to prevent multiple entries are not set, too.

They use Serbian lattin, probably "" etc.

Version: Version 3.15.1+181017

Hi, LeeLoo,

what do you expect us to do?

We do not know your survey? So, please provide it as *.lss.
What is empty?
The whole answer record is missing?
Or: only the data of the questions, but things like "id", "seed", "complete", "startlanguage" are there?
I am sure you can identify the copies which were not entered.
There should be a variable which identifies the person who entered. Then you can see which of his ten questionnaires is empty.
Did you try to enter these again, maybe without real question text, only with placeholders?
If this works you can enter step by step the real text to see where and when the problem occurs, if it does.

or there are answers only to the requested questions

Are you sure that there were data entered in the not mandatory questions?

Joffm