Adjust how the command line is build, the function

handle_witness_of_task always returns a list of input_files

benchexec/tools/metaval++.py

@@ -52,7 +52,7 @@ def cmdline(self, executable, options, task, rlimits):
             task, options, "--witness", TaskFilesConsidered.SINGLE_INPUT_FILE
         )
 
-        return [executable] + options + witness_options + [input_file]
+        return [executable] + options + witness_options + input_file
 
     def determine_result(self, run):
         separator = ":"