Finally I've got the problem solved. Some low-level corrections had to be done on R77.30 database before importing to R80.10.
Like I told previously the file $FWDIR/log/cpm_for_cpdb-YYYY-MM-DD-HH-MM-SS.ELG is very important as it points to details.
1) In this case there were syntax errors ( double brackets '}}' ) on a couple of files, namely sem_event_views.C and sem_views_collections.C. Solution is to correct the syntax by eliminating the redundant bracket.
2) Also there were some null references on some objects of anti_malware_rulebase_rules table. One can either try to delete those objects (through guidbedit) or recreate the antimalware policy from scratch on smartdashboard, whatever suits best.
3) However, after that I also needed to run a script (CleanMalformedEntities.sh) as per sk117167.
Et voilá ! It worked. I must thank TAC for the support provided.
Just one last remark:
I've been doing several R80.10 upgrades recently and I've been successful and autonomous in almost everyone (>90%).
Regardless of that, I must say that the problems encountered here were not manually induced by humans and I must say that the upgrade process / verification tools should take care of it, and therefore it should improved.
I guess/suspect this might been caused by very old databases that were subject to several upgrades along the way.
Regards,
PB