You have got me on the right track!
session.save_path in php.ini was pointing to a non-existant folder!
A partial success:
I reached Step1, but:
Converting...
Converting membergroups... Unsuccessful!
This query:
INSERT INTO `smf`.smf_membergroups
(groupName, onlineColor, minPosts, stars)
VALUES (SUBSTRING('Forum Founder', 1, 80), '', -1, '';
Caused the error:
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 3
But thats better than nothing, isn't it?
The YaBB forum was heavily modded, so there could be some incompatibilities...
I will now look into the converter code to see if I can fix this issue.
I'll keep you informed!