I got the same message from 2 different servers running Debian 9 today.
Searching the forums I came across this thread:
Resolved - SpamAssassin daily update cron exited with return code 4
The output of the /usr/bin/sa-update; echo $? command is this:
config: invalid regexp for __URI_TRY_3LD 'm,^https?://(?:try(?!r\.codeschool)|start|get(?!\.adobe)|save|check(?!out)|act|compare|join|learn(?!ing)|request|visit(?!or|\.vermont)|my(?!sub|turbotax|news\.apple|a\.godaddy|account|support|build|blob)\w)[^.]*\.[^/]+\.(?<!list-manage\.)(?:com|net)\b,i': Variable length lookbehind not implemented in regex m/(?i)^https?://(?:try(?!r\.codeschool)|start|get(?!\.adobe)|save|check(?!out)|act|compare|join|learn(?!ing)|request|vi.../
channel: lint check of update failed, channel failed
It semms that the __URI_TRY_3LD rule has wrong regexp?