This is a slightly odd one so please bear with me. It concerns a 2.2 block causing Moodle 2.4.1 to partially fail.
Also, Dan Poltawski sent me! https://twitter.com/SDCMoodle/status/294006181779881984
Expected behaviour: blocks install without error, administrative functions not affected.
I am installing plugins en masse into a fresh copy of Moodle 2.4.1. I have discovered that after installing a particular block (the GSB block, found here: https://github.com/havingar/block_gsb), the 'check for available updates' button within Moodle works, but produces the 'Error output, so disabling automatic redirect.' warning, but with no error either on the screen or in Apache's error log. This is with full developer-level debugging on and displayed. (I realise this block is for Moodle 2.2 or greater, and may not be 100% compatible with Moodle 2.4.)
Saving changes to any other admin page throws up this error too, but again, with no actual error. Removing the block solves these issues. I've isolated the GSB block as the problem by installing blocks (plugins, themes, grades, question types etc) one at a time and then testing the 'update' function before moving on.
I would provide an error report, but even with developer-level logging on, there's no error to be found anywhere.
I have mentioned this in a GitHub issue to the developer of the block but was additionally asked by Dan Poltawski to report this here too.