This is exactly the same as here: https://ourworldindata.org/coronavirus/country/serbia

So I'm assuming that this dataset can be trusted.

The point:

From the dataset: Date: 2021-12-16, new tests: 13690.0, total tests: 7032035.0 Date: 2021-12-22, new tests: 14808.0, total tests: 7107851.0 Date: 2021-12-26, new tests: 9265.0, total tests: 7158932.0

As We can see Djoko's "ID" (7371999) is much larger than total number of tests for 26th of December 2021 (by 213067) and for me this furthers your point that We don't know how this number is generated. (It seems to be sequential, but is it?)

All invalid reports ("Report IS NOT VALID!") are in red color (Bootstrap 3 class .alert-danger). You can see it when open https://pcr.euprava.gov.rs without any subdir/script/query string.

This green for positive test mean just that QR code and associated test are valid, not that it’s good or bad. Positive test is bad in the first 14 days but after that it’s good - it gives you same rights as if you were vaccinated. But it’s the same test in both cases and green just tells us it’s a valid document.

Digital certificate is different kind of document. That’s the one you need to enter restaurans, bars, etc. after 20h - it will be red if you don’t fulfill conditions, e.g. vaccination or positive test within last 6 months (but not within 14 days), etc.

In programer’s lingo, test result is const, while digital certificate is a function of multiple conditions in context of current date/time.