I would still like to know ‘HOW’ you are attempting to edit the post… via the edit button or the edit link in the forum tools??  and if you have tried both of them…

also, did you try increasing the post_max_size again?  editing a post will ‘post’ more data than creating the post because of additional date that has to be posted…

Using the edit button. The one at the top right with ‘Delete’, ‘Edit’, ‘Quote’. 

have tested and not able to replicate this, of course… 

do you get the same issue if you use the forum tools edit link?  and what about another increase the post_max_size?