Progress of mongodump operation jams at a random point

We are executing remote mongodumps to backup collections. For some time now sporadically one of these dumps stops to progress any further and remains to log out the same amount of documents processed over and over: 2016-06-27T06:35:06.428+0000 [##############.......] col 1140198/1853120 (61.5%) 2016-06-27T06:35:09.428+0000 [##############.......] col 1140198/1853120 (61.5%) 2016-06-27T06:35:12.428+0000 [##############.......] col 1140198/1853120 (61.5%) This time the dump is stuck at this point for about four hours and more than 5000 log lines showing 61.5% of progress. I can still access the MongoDB Server from the backup machine, mongodb answers to queries, no annomalies in the logs. MongoDB version 2.6.12