You can find the DB schema upgrade scripts under the upgrade directory. The subdirectory name is for the new jforum version where database schema is also changed. So, you'll need to go through each subdirectory from 2.1.5 to 2.8.0.
Using this as an example for mysql:
mysql -u USERNAME -p DBNAME < 2.1.5/mysql_2.1.4_to_2.1.5.sql
You may try to disable cache by adding the following setting to jforum-custom.conf and see if this works
#########################
# Topics, Posts and Cache
#########################
topic.cache.enabled = false
posts.cache.enabled = false