Had same issue. Try logging out of zammad, clear browser cache, close it, reopen browser, log in to Zammad and go directly to Settings -> Knowledge Base. Activation should work.
I think I found the Log file in /var/log/zammad/production.log
There is an interesting part in it:
I, [2019-06-07T14:46:45.939620 #425-69901371146260] INFO – : Processing by KnowledgeBase::ManageController#init as JSON
I, [2019-06-07T14:46:45.939677 #425-69901371146260] INFO – : Parameters: {“_”=>“1559911236338”}
E, [2019-06-07T14:46:45.946228 #425-69901371146260] ERROR – : PG::UndefinedTable: ERROR: relation “knowledge_bases” does not exist
LINE 1: SELECT “knowledge_bases”.* FROM “knowledge_bases” ORDER BY …
^
: SELECT “knowledge_bases”.* FROM “knowledge_bases” ORDER BY “knowledge_bases”.“id” ASC LIMIT $1 (ActiveRecord::StatementInvalid)
Does that help you?
It seems the database could not be updated properly