Transport imported successfully:

0: Transport successful

4: Warning occurred

Transport did not import successfully:

6: Post-processing is required

8: Transport carried out with an error [individual object could not be transported successfully]

ex: objects could not be overwritten

Transport termination:

12 and greater: transport was terminated

The fist and most obvious course of action is to contact your Basis support team.

Given that you are reading this article you probably wanted or were forced to figure this one out on your own so here are a few helpful actions that you can take to narrow down the problem:

TP processes:
Go to the SM51 and check if any of the TP processes are still running, you might be required to kill these before proceeding haulted imports.

Check the TP System Log:
You can do this by STMS – Overview – Imports – Select Queue System – Goto – tp system log – F5
This will show you a step by step log of what has occured in the system, you would be able to see which transports are being processed and what has completed.

Check the Import Monitor:
You can do this by STMS – Overview – Imports – Select Queue System – Goto – Import Monitor
This will show you the queue of active transports [if any], if you see that there is a collection of unprocessed imports then you need to flush the queue.

Check TMS background job:
You can also check if the background job RDDIMPDP is running.

From tim to time it happens that the import queue becomes inconsistent and the system stalls, as a result you can be stranded without any of your transports moving ot he next system.

Tables:
TMSTLOCKR: will unlock a given transport from the transport queue [the transport truck will dissapear after deleting of the entry int he table]
TRBAT: will contain values correspondign to active imports, if you have a stuck transport that is haulting a collection of other transports a simple delete from this table will re-initialize the queue and push the backed up imports forward.
TRJOB: should also be cleared in order to continue the regular flow of backe up transports

For any additonal information please refer to the following notes:
818065, 302859, 556941, 556946, 71353, 49242, 323726, 56311, 24800, 486991, 506771, 138200

Useful transport tables [listed above].