Did you recently upgrade from 3.0.highest to 3.1.highest?
Actually, that might not matter but would help me verify what I found doing an upgrade as above.
Beginning version 2.9.x Moodle 'transitioning' away from the YUI library. YUI library might be used in filepicker.
https://docs.moodle.org/dev/YUI
Is your site set to combo load YUI libraries?
Try this in config.php of site:
$CFG->useexternalyui='0';
$CFG->yuicomboloading='0';
Check your DB - mdl_config table to see any settings for YUI external or combo loading.
Purge server caches ... might have to do that manually to assure
caches have been eradicated.
IF the above works, suggest making sure that YUI settings stay '0'
by editing the mdl_config table and setting the above variables to '0'.
You could then remove the config lines.
'spirit of sharing', Ken