Hi again,
I've found out what went "wrong", the default behavior of Logstash file input plugin seems to be ignoring files that are older than 86.400 seconds (24h) which explains why changing dates of the old files with touch filename.txt worked.
This seems to be the default behavior for Logstash 2.2.X? as I've only observed the problem with Logstash 2.2.x.
Found this out by running debugging on Logstash with logstash -f /path/to/config.conf --debug, which pointed me in the right direction.
See documentation for details
I've now added ignore_older => 0 to the file {} configuration and importing of older files is working as intended again.
Hope it helps if someone else is struggeling with stalled imports of files over 24h.
Cheers,
Kevin