Import function not working when changing dat file

At beginning I was not able to import succesfully any exported dat file.

One day later import works as intended. Was it bad luck yesterday?

But if I make any modification in exported “.dat” file via Notepad++, I get error during import:
“Error uploading settings! Please check that you uploaded a customizer export file!”

I need to change one part of existing url(in images paths and links) in all section and items inside customizer and I do not want to write it manually.

Export / Import function seems to be good way to find/replace it in one step…

Any Ideas how can I edit a dat file without damaging it?

Thanks

Tomas