For some reason, the Navigation side block is not displayed any more at moodle.org/plugins. This has significant impact on the whole UI as it relies on the block being displayed a lot (see https://lang.moodle.org/mod/forum/discuss.php?d=3969 for example).
I don't know what could cause this. Initially I though it was a regression of
MDLSITE-3393 but it does not seem to be the case (the block is neither displayed with the previous layout file).
What I tried at next.moodle.org was to change the block's default position to the "Right" instead of "Left" (as we use single side column anyway) and it seems to help. But I don't want to perform this at the live site without Helen Foster's attention.