In line #103 you have passed $value to htmlspecialcharacters after you have set $value as an array earlier, before line #94 or at #96.

htmlspecialcharacters is expecting a string, not an array, as the first argument.

You need to change #103 to be the same as #100 and pass it $val and not $value.

In line #103 you have passed $value to htmlspecialcharacters after you have set $value as an array earlier, before line #94 or at #96.

htmlspecialcharacters is expecting a string, not an array, as the first argument.

Updated:

To be executing line #103 you have failed to meet the condition on #94, $multiple is True and $value is an array. The error message says that $value is an array therefore $multiple must be false.

I attempted to recreate your described scenario of creating a menu item to create an article and I can confirm $multiple is false by default but I can't cause the message because $value is a string and not an array.

The $multiple is set by a field in the .xml file for the form or custom field that has the attribute of multiple="true" and for $value to be an array then the field is probably a drop down list or checkbox field type with more than one value that can be set.

Given I can't recreate the error with a standard Joomla install then is it possible you have custom field(s) added to the category or article or an override of the article/category form that has a new or modified field that is missing a multiple="true" statement?