Hi @peznan,
I’m not sure what could be causing the issue as there are no explicit errors given by openrouteservice, which suggests that it is some other issue. The file size shouldn’t be a problem as you have set the RAM as you have set the limits in the JAVA_OPTS line correctly.
A couple of things you could try is to delete the graphs folder on the host and change the BUILD_GRAPHS value to be false - this would tell it only to rebuild the graphs if there is nothing present in the graphs folder… This would help if it was a file permission issue
The other one as you said Leichenstein was ok would be to try using osmosis for the merging as I have used that in the past without problems. Once installed, the command for that would be osmosis --rb germany.osm.pbf --rb france.osm.pbf --merge --wb D_FR.osm.pbf… That would help if the osmconvert tool is making things strange.
The other question that might help is how long it takes before it exits? Is it straight away or after some time?