Hello Anton,
That's strange. Unless the requests were hard-deleted(via code or manually) from the header table I don't see a reason as to how this could have happened.
Now for the solution to your problem, you might have to remove all R3AR* queues in SMQ1 and SMQ2 transactions on both CRM and ERP side. But exercise due caution before deleting these R3AR* queues and check if some of these have been initiated lately.
Then reset their statuses in SMOFRSTAT manually.
Then retry.
BR,
Rohit