My guess was right and the mystery is solved – the plugin inserted this
Strict Standards: Redefining already defined constructor for class wpide in .../dev/wp-content/plugins/wpide/WPide.php on line 31
Strict Standards: Redefining already defined constructor for class wpide in ../dev/wp-content/plugins/wpide/WPide.php on line 31
into loop-staff.php and framework-extender.php file when I edited using the plugin. Hence, we were seeing the error only for Invent theme and not for Twenty Fifteen.
Regarding the changes, I am submitting an update today with the changes. Pls wait on the same. You can also download the two files from the old installation, compare and patch new one if you are in a hurry. I will now have to search for a replacement for wpide which had served us well so far.