How to debug PHP to solve WP export file import error

when importing a wordpress export file, it shows a popup asking to save admin.php.

this problem was there for 3 days.

but today it worked again, without doing anything (well I just updated from PHP version 5.3 to 7.0).

It worked only once for it15, then not working more again.

I changed the Theme to Twenty Ten, does not help, so the problem is not caused by the Theme.

BUT:
If export the category W in it15, then it is a very small xml file, then import in wautom OK:

OKAY:
I think there is just one or more special POSTs which is causing the problem.

Now my solution is:
When exporing, divide the export to a certain period, e.g. From Jan 2015 to Augustus 2015, then importing will be OK.

OR:
Is it because of Unicode (Chinese text?)
Should not be this reason, because W category had no problem.

OK: I have more inside of the problem:
1. If I export from Aug 2016 to Oct 2016, then import OK
2. But If I export from July 2016 to Oct 2016, then import NOT OK
3. But If I export from July 2016 to July 2016, then import OK
==> the size problem?

OK: My Final Workaround is:
Export each month, then import always OK.

ALSO: good trick
Sometimes just click that Submit of the Import button several times each after a while, then it might work.