SOLVED!
In a sort of weird way: After re-installing libreboot, Trisquel and
libre-linux - with no effect - I tried to re-configure a different key (left
side logo-key). That worked, and re-re-configuring to use the key-pad enter
once again, it now works as intended.
I have had issues with '@' earlier and never solved it the last time. Below
is a copy of an advise proposed by Eemeli
(https://trisquel.info/en/forum/librebooted-and-trisquelised-macbook21-where#comment-123269)
According to that (and earlier comments), re-configuring the keyboard should
