Hey @Canapin
When we migrated our old vB3 forum we had a lot of errors in the migration script when some strange mojibake or some odd-ball attachment that was polluting the DB from 15 years of posts.
What I did, not very elegant, but it worked for us:
When we ran into these errors, I simply edited the migration script where the errors happened; and wrapped the offended line with
begin
#offended ruby line here
rescue
puts "here is some interesting offending information"
end
My experience with migrating around 1 million vB3 posts to Discourse is that one of my best friends was:
begin
rescue
end
We lost a few malformed posts, but that was OK by me to drop a handful of posts out of a million.
Hope ths helps.