Oplog incremental backup failure
You are right, the oplog truncation is killing your cursor.
There are no indexes on the oplog, so dump has to scan from the very beginning to find the documents that match the query.
In that case of this mongodump cursor, it had examined 356596 documents, but still hadn't found any that matched the query.
The failure message reports the last seen record was &quot;RecordId(6850019249918838732)&quot;.  That number is a 64-bit value that corresponds to the ts.
You can get the first 32 bits and the corresponding timestamp from bash:
% echo $((6850019249918838732 &gt;&gt; 32))
1594894391

That date is Thu Jul 16 10:13:11 UTC 2020 still over a month prior to the first date of interest.
When those documents were trimmed from the capped collection, the cursor no longer pointed to a valid position, so it was not possible to continue.
When that happens, you'll need to restart the mongodump so it can start scanning from the new beginning of the oplog.