Hi @Hasenlaeufer, thanks for taking the time to report this!
It's always very welcome to explain steps to reproduce, expected outcome, and actual outcome.
If I understand correctly, this report is about one single video showing "Error" under "Transcode status" for "WebM 480P" and "WebM 720P"?

Exitcode: 124

T157027 may be related

I ran the transcode locally and on encoding01:

Adding the defined / expected ulimits (cpu time, mem, filesize) (1 2 3 4) did not reproduce the issue:

But interestingly, there's a wall time limit that's the minimum of the cpu time limit and configured walltime limit, which has a default of 180 seconds. Seeing a walltime limit successfully reproduced the error:

This is consistent with the timestamp data in the database (3 mins encode time):

MariaDB [commonswiki_p]> select transcode_time_addjob, transcode_time_startwork, transcode_time_error from transcode where transcode_id = 2484549;
+-----------------------+--------------------------+----------------------+
| transcode_time_addjob | transcode_time_startwork | transcode_time_error |
+-----------------------+--------------------------+----------------------+
| 20171016152933        | 20171016152935           | 20171016153235       |
+-----------------------+--------------------------+----------------------+
1 row in set (0.01 sec)

1bb9a22 by @MaxSem changed the fallback behavior from actually falling back to the minimum.

Change 384624 had a related patch set uploaded (by MaxSem; owner: MaxSem):
[mediawiki/core@master] Better walltime fallback

https://gerrit.wikimedia.org/r/384624

@Aklapper & @zhuyifei1999: Thanks a lot by taking care of this task!

@Aklapper: You provided a link to a help page at mediawiki.org, titled "How to report a bug". Thanks for this hint! Up to now, it's not easy for me to understand the relationships between "mediawiki.org", "wikimedia.org", "commons.wikimedia.org" and "phabricator.wikimedia.org". Maybe, it's better to clarify this theme at another place, because it is not related to this task. Do you have an idea, where we could continue this theme?
Did I miss to provide a needed detail?

You wrote: If I understand correctly, this report is about one single video showing "Error" under "Transcode status" for "WebM 480P" and "WebM 720P"?
The answer is: Yes & No. In detail:
Yes, the source is a single mp4 file.
No, because I mentioned 3 different WebM files. They are based on a commons single mp4 file, but the 3 different WebM files are created by 3 different video converter tools.

@zhuyifei1999: Thanks a lot for your efforts to investigate this theme! I don't unterstand the technical details of your investigations.
Please, can you summarize the results of your activities? Did you detect a bug or a wrong configuration? What does it mean: Can the problem be solved by changing a configuration setting?

In T178314#3689250, @Hasenlaeufer wrote:

@zhuyifei1999: Thanks a lot for your efforts to investigate this theme! I don't unterstand the technical details of your investigations.
Please, can you summarize the results of your activities? Did you detect a bug or a wrong configuration? What does it mean: Can the problem be solved by changing a configuration setting?

No. The configuration sets a cpu time (the sum of the time usage on all cpu cores) limit of 16 hours. Previously, the wall time (the total time spent) limit, when unset, is the same as cpu time, but now, when unset, it is the minimum of 180 seconds and the cpu time limit. This effectively limits the wall time limit to only 180 seconds, which is way too small for such transcode. The transcodes fail due to the timeout.

The fix would either be the above patch, which makes a cpu time limit setting also the the wall time limit, or make TMH transcode increase the wall time limit to cpu time limit explicitly.

Change 384624 merged by jenkins-bot:
[mediawiki/core@master] Shell\Command: Better walltime fallback

https://gerrit.wikimedia.org/r/384624

Change 384647 had a related patch set uploaded (by Legoktm; owner: MaxSem):
[mediawiki/core@REL1_30] Shell\Command: Better walltime fallback

https://gerrit.wikimedia.org/r/384647

Change 384647 merged by jenkins-bot:
[mediawiki/core@REL1_30] Shell\Command: Better walltime fallback

https://gerrit.wikimedia.org/r/384647

Change 384790 had a related patch set uploaded (by MaxSem; owner: MaxSem):
[mediawiki/core@wmf/1.31.0-wmf.3] Shell\Command: Better walltime fallback

https://gerrit.wikimedia.org/r/384790

Change 384790 merged by jenkins-bot:
[mediawiki/core@wmf/1.31.0-wmf.3] Shell\Command: Better walltime fallback

https://gerrit.wikimedia.org/r/384790

Mentioned in SAL (#wikimedia-operations) [2017-10-17T23:22:07Z] <thcipriani@tin> Synchronized php-1.31.0-wmf.3/includes/shell/Command.php: SWAT: [[gerrit:384790|Shell\Command: Better walltime fallback]] T178314 (duration: 00m 51s)

Great! I did not expect, this problem could be solved so quickly. Thanks to all who have been involved in this issue and have helped to solve this problem!