Warning threshold deleted if check_by_ssh

Hi all!

We have a very strange problem…
We manage some server, where we cannot install Icinga2-Agent nor NRPE and we must use check_by_ssh…
We are not root on these servers.

So, if I start this command on the server:
/data/opt/current/common/nagios-plugins/check_disk -u MB -W 15% -K 10% -w 15% -c 10% -p /data -f
it works and I get the peformancedata back:
DISK WARNING - free space: /data 103780 MB (10,18% inode=65%);| /data=914902MB;865880;916814;0;1018683

If I try to start the same check via SSH from our Icinga2 server:
ssh custadm@x.y.k.z -p 22 -i /path/to/our/private/id_rsa /data/opt/current/common/nagios-plugins/check_disk -u MB -W 15% -K 10% -w 15% -c 10% -p /data -f
it works too:
DISK OK - free space: /data 202749 MB (97.54% inode=100%);| /data=5112MB;176682;187075;0;207862

But in Icinga2 there is no warning threshold…
So I activated the debug module and I see which command will be used:
'/usr/lib/nagios/plugins/check_by_ssh' '-C' ''/data/opt/current/common/nagios-plugins/check_disk' '-K' '8%' '-W' '15%' '-c' '8%' '-f' '-p' '/data' '-u' 'MB' '-w' '15%'' '-H' 'x.y.k.z' '-i' '/path/to/our/private/id_rsa' '-l' 'custadm' '-p' '12202'
and now the warning threshold (and just this!) will be deleted:
DISK OK - free space: /data 103641 MB (10.17% inode=65%);| /data=915042MB;;937188;0;1018683

Any idea where could be the problem?

Thanks a lot