Error in file handling

This has probably to do with underscores ( _ ) in variable/method names. (not allowed in object script, but used with callbacks in the (deprecated) product VISM (or also called Cache Direct, hence the CD and CalBk in the error)
I get a similar error when doing :

%SYS>set _abc=1
 
<FUNCTION>SetProp+2^%CDCalBk
%SYS 3d1>

From the documentation :

Invalid Names

A local variable name that does not follow the above naming conventions generates a error. There is one exception: if an invalid variable name begins with an underscore character followed by a letter, the system generates a <_CALLBACK SYNTAX> error. (Note the underscore character within the error name.) For example, SET _abc = 123 or SET x = _abc. This is because Caché identifies these names as VISM control names, rather than local variable names.