MetaIO supports One-Slice-Per-File Data Formats and it uses the % character to specify the slice number in the filename format string.
Due to the special meaning of the % character, currently MetaIO does not allow using this chracter in the path. This limitation should either be clearly documented or the implementation should be made more sophisticated (to somehow differentiate the % that refers to per-slice filename generation from simple % occurrences in the path; or by allowing completely disabling this extremely rarely used one-slice-per-file feature).