Error notification always come out when open Buy & Sell menu at Miroku's shop causing to return to login screen. No problem with Bargain.
My Miroku is lv27 with lv8+1 Shop Inventory

Edit:
The bug has been fixed after 7/3 update.
http://onigiri.cyberstep.com/info/topic/update/19716

Good to know it is not just me being hit by this error. Unfortunately I have exactly the same partner levels, so no idea if it is for all Mirokus or just those at this affection stage.

Tested with my 2nd character which still have lv1 Miroku. I able to open Buy & Sell menu like usual.
So the problem is most likely happen on specific Shop level which current known lv8(+1)

Now just wait if they notice this problem or not.

Yes. Cyberstep, please fix this issue. Its irritating when you are trying to sell things.

same with me but I am at lvl7 (+1) shop inventory

IT seems that anyone who has their Miroku at level 20 and above has this bug. 19 and Below is safe and does not produce this result.... And according to a post in the General Discussions about this issue, they are aware of it and will fix it "soon". So, don't hold your breath for much in at least the next couple days....

I have a friend who recently level his Miroku to lv20 with Shop Inventory at lv 7(+1) and he immediately got the shop bug.