Ich denke, ich habe das Problem gefunden. Hab folgendes in der Knowledgebase von FTDI gefunden:

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Why do I get repeated data when I read from the FT245BM?

The WR and RD pins may be driven too hard, which can cause ringing on the lines. This can result in corrupted data because WR and RD are edge sensitive. Fitting series resistors to the WR and RD pins in the range 20-40W close to the microcontroller should remedy the problem.
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Nachdem ich nun 24 Ohm in die WR-Leitung eingbaut habe scheint es zu funktionieren.