ABEND USER COMPLETION CODE=0301

this abend is generally related to a problem of size of the BUF1SP parameter (GENDEB).
Parameter BUF1SP is related to the activity of InterPel, in particular as regards parallel transfers.
You have to exceed the threshold.
The value by défault must be at 1000.
To see whether this parameter is coded in your parameter setting and which is its value. If the problem occurred only once, you can make +500, if that often occurs, make +1000

In the documentation  :

BUF1SP={nnn|1000}
Size in Kbytes reserved for Inter.Pel inter-task communications.
Default: 1000.

About the BUF1SP :

The size of BUF1SP is concerned by InterPel activity. All messages exchanged by the 3 "Addresse Space" pass through the buffer.
The activation of the "Sentinel" option increases the number of messages processed by InterPel and therefore has an impact on this size.

To calculate the size of BUF1SP, it is important to know:

 -- The maximum number of transfers that can be active simultaneously.
 -- The number of transfers to Send (E9) waiting in the mailbox.

In which case the BUF1SP may be too short;

 -- If InterPel or the network were unavailable and that the Mailbox contains many transfers in wait, at the relaunch, InterPel is going to need a lot of memory in the BUF1SP.
 -- If you deposit in the MailBox many transfers as server mode (FTP, for example) InterPel creates entries in the BUF1SP