Buffer raises error on any unit except decimal degree

Hello dear kind GIS Users,

i recently have trouble with the Buffer UPDATE: EVERYtool, when I want to buffer/erase/anything based on Units a Shape File Feature Class by 1000 Meters the Buffer Tool raises the error 999999. It is raised with any Unit Setting except Decimal Degrees.
Its the same when i try to do it via the python console. I think it has something to do with the values beeing processed as Strings, since i think there was some kind of string error raised, but now

Code:

import arcpy

arcpy.analysis.Buffer("landuse","landuse1000","1000 Meters")

does raise the error 999999

if i type it with no "" as a number

Code:

import arcpy

arcpy.analysis.Buffer("landuse","landuse1000",1)

The Buffer is processed but obviously with a way to huge radius.
Same goes with the Toolbox, nothing works except Decimal Degrees.

As I found out recently it the same with any Tool based on Units...

I don't know whats going on, I mean I get the hint with the string vs. integer, but I can't imagine that the buffer script is somehow corrupted. I would gladly appreciate if anyone can give me a suggestion what to do or a hint where the error is produced :)

Thanks in advance