Mainly for @elkuku since he wrote this (and only a couple of us can see this anyway)

https://issues.joomla.org/config#log

The "levels" key is throwing an array to string conversion error, looks like the UI wasn't accounting for possible nested values like this. Both the top level "log" and "cache" keys can support nested values which would result in nested arrays like this. Until today, the nested log key log.levels.application wasn't in use and this is the one breaking it (seldom used feature that lets each individual logger have its log levels configured).