What happened:

A trigger prototype on a template was moved from one LLD rule to another by changing the item prototype referenced in the trigger expression. This process took a long time, and caused issues by locking the database for a long time. The process was eventually killed to release the lock again.

The move seems to have worked, we see the trigger prototype on the LLD rule we moved it too, and hosts have the trigger discovered. When trying to remove that trigger prototype, we get the following error message (following a list of triggers it tries to remove): http://hastebin.com/raw/ruwekupani

According to gleb on the IRC channel, moving prototype triggers from one LLD rule to another is prohibited on newer versions (ZBXNEXT-2118). He also suggested we can as a work-around split the query in two, first to delete the child triggers, but we would like some advice on that.