It looks like there is a bug in one of your add-ons that uses hook location
cache_templates
Code:
array_merge(): Argument #1 is not an array
...
array_merge() in includes/class_bootstrap.php(1422) : eval()'d code ...
Since you mentioned that you recently upgraded vBulletin 3 to vBulletin 4, it's possible you are still using vBulletin 3 code in one of your add-ons that uses this location.
You must fix or disable the broken add-on.
Otherwise, if there is a compatibility issue, I will need to know the code from affected plugin.