Thank you, removing line breaks indeed solved my problem.
Yet I had to take one more step inbetween due to the previous (failed) import attempts. I tried to do as it follows:
1) Delete the corrupted/failed import on the target WP installation
2) Edit the original message field on the source WP installation in order to delete line-breaks
3) Save
4) Export as XML
5) Import back
But it didn’t work: even if I was actually importing a “line-break” free message field, either no fields were created on the target WP installation (as result of the new import procedure), or only one message field would show up… with line breaks.
So I cloned the original fieldgroup (on the source WP installation) with Duplicate Post (http://wordpress.org/plugins/duplicate-post/), exported it, imported it back and… it worked.
