Jump gotcha ... is ltiprovider in your 3.5 an addon? ... probably is ... located in code/local/ directory.
Try this:
backup db
move code/local/ltiprovder directory to some other location.
check your mdl_config_plugins table for any references to a local ltiprovider addon ... remove them.
Assumes you are not using ltiprovider.
https://moodle.org/plugins/local_ltiprovider
local ltiprovider is supported up to 3.0 of Moodle
So you probably were not using when site was a 3.5.
https://moodle.org/plugins/pluginversions.php?plugin=local_ltiprovider
'SoS', Ken