We used to see this problem fairly frequently when we were using the command line to send all transports, but when SAP unveiled STMS this problem all but disappeared. That is until we saw it again this week. We had a scheduled transport job that started about 1 minute before we had to take down a system for emergency hardware maintenance. When we brought it back up, lo and behold the little truck was still showing on the front of our system transport queue and we could not send any new transports to that system. "TP Import is Running"
So I had to dig into the old file cabinet to find the cure.
Inside the DB there is a table called TPSTAT. This table contains information about import requests in the system. If any row has a value of 'R' in the STATUS field, TP is rendered inoperable. Change the value to 'F' or delete the row altogether in SM30 or through a SQL query at the Database level and you will be ready to go.
