Some translation services are more picky about the format of the .pot file than others. I encountered two issues when trying a different service:
1. Add compliant header
More about headers in this paper  and the gettext documentation . I've successfully managed to upload a .pot file to a translation service by adding this header:
2. Escape quotes
Some services can't deal with quotes in the translatable string:
One could argue that is this particular example the html should not be exposed to the translator but it's imaginable that quotes are used in strings that are presented to the user.
Escaping with a backslash allowed me to upload the .pot file: