In parseVariable function is taken incorrectly the context variable:

if (varName.charAt(0) == '#') {
varName = varName.substring(1);
val = getCompiereService().getCtx().getProperty(varName);
}

For example #AD_User_ID => AD_User_ID

The solution:

if (varName.charAt(0) == '#') {
val = getCompiereService().getCtx().getProperty(varName);
}

This correction is applied in the filter parameter of the web service.