Most of the time import fails due to invalid characters, wrong coding or too long string in your csv file.
In order to investigate, open your import history in the contacts menu:
Check what line has failed:
We can see, that from 15034 lines our problem is by line 2125 + 12575 = 14700. So we would start the investigation from line 14700. And that would be wrong. Failed lines and other import errors wouldn’t generate insert and import, but error. And that is not shown here.
But if we manually run it, we can see the problem in the backend:
Look up that line and you’ll find some irregularities. Too long strings can for example block the import. Like:
Once you removed the problematic lines, you can continue the import: