Probably due to an old block: \blocks\fn_my_menu
Please see:
https://moodle.org/plugins/pluginversions.php?plugin=block_fn_my_menu
which shows compat up to version 2.5 of core code Moodle.
Fix (?) is to manually remove the block ... code directory and any rows in mdl_config_plugins related to that block.
'SoS', Ken