There will be an update coming for handling arduino in the near future once it gets pulled in and is a part of the future releases. The current implementation is buggy and outdated the updates are here:
https://github.com/DomAmato/openFrameworks/tree/ofArduino_Updates
You should be able to just replace the files in the libs/openframeworks/communication folder with the ones found in that branch (while it still exists, one it gets merged it will be deleted) That means replacing ofSerial and ofArduino files