Hi @Marthael,
are you using “KNIME Column Storage (based on Apache Parquet)” as your Row-based Table Backend implementation? It’s not actively maintained anymore and should not be used. The default Row-based Backend or Columnar Backend (which does not use Parquet), should be used.
Does using the default Row-based Backend implementation or uninstalling the extension outright solve the problem? In both cases, I think you have to reset the full workflow, though 
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Don’t worry, the Parquet Reader and Writer are part of a separate extension “KNIME Extension for Big Data File Formats” and are unrelated to your error.