Hello, I'm facing the same error.
We have decoupled scenario on SRM 7.13 EHP 3 ALSO note 2038377 implemented both on sell-side and buy-side. When purchaser change submission deadline and end of bidding period in future, bidder still can't send response, getting error End day has been reached. Submission deadline passed. In header it is countdouwn, which shows, that it is still some time to send response. Did you get a solution?