Since API catches exceptions, it is impossible to debug them. I have enabled exception logging and backtraces, but only thing in php error log is the following:

2012-10-29 18:01:04 translatewiki.net mediawiki-bw_: [43eeba39] [no req] Exception from line 544 of /www/translatewiki.net/w/includes/content/ContentHandler.php: Format json is not supported for content model wikitext

In addition the http requests are served with HTTP 200, which makes it impossible to grep the access logs for causes.

Version: 1.21.x
Severity: major