Hi,
I have not been able to recreate your issue, I need to check something and need more description to be able to pinpoint the cause. I dont know how much you know about html/php – I will try an explain, where the problems occur, that might help us to find the root cause.
I believe it is this line that fails:
<option value="<?php echo $category1['cat_ID']; ?>" <?php if ( $cats_selected && in_array( $category1['cat_ID'], $cats_selected ) ) { echo 'selected="selected"'; }?>><?php echo $category1['cat_name']; ?></option>
and this function in_array( $category1['cat_ID'], $cats_selected )
It is the cats_selected that is not an array, meaning there hasnt been selected any category (or the format of one of the categories messes up the array)
So to my question: When does this happen ? Is it when you try to create a new post or when you edit an existing ?
Have you set a default category in frontier post settings for all profiles ?
The cats_selected should never be empty…
Can you try and change the name of you category
Member′s cars
to Members Cars ?
As for your question on formatting, can you link to a screenshot, and maybe also a screenshot form when you get the error