Solved it:
https://dev1galaxy.org/viewtopic.php?id=5509#p40818
TL;DR: I had a backported librtmp1 way back in Debian 8 which never got upgraded, because of being later than the versions of Devuan 2 and 3 delivered. Devuan 4 actually had a newer one, but because a different - higher - ASCII character separated the equivalent version number from the date, APT thought it, the backportad one is the higher. So, the solution was to
downgrade librtmp1 from the
older to the
newer, from
2.4~20150315.gita107cef9b-dmo1+deb8u2 to
2.4+20151223.gitfa8646d.1-2+b2.