Synology - Free Memory problems

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Hello everyone,
I've currently installed cacti on my Synology DS1515+. All graphs look fine, except the "Memory free" one, I'm not getting any data on that.
As I'm not that good with perl scripts I didn't find a solution myself.
My Synology is running DSM 5.1-5022 Update 3.
When I look into the Debug Graph mode it says "RRDTool says OK" and I can see the .rrd file in the folder (which got 777 permissions).
I already tried to change the line "perl" to "/opt/bin/perl" under Data Input Methods, but that didn't help either.

Here are two screenshots of the debug page and of the Data Input Method.

If I should add more please tell me

Thanks in advance,
Marco