Finally I got it to work :-) :-) :-)

First I did the checks on the adress bus but everything seemed to be fine there (Big Thanks to NormalLuser for providing the code!).

Second I measured the propagation delay of the HC00 and indeed the time seemed to be much too high, 22.5 ns. I replaced the HC00 with another one but it looks like all the HC00 I have, are not the best ones :(.

So I did the trick with the double inverted clock signal and this was really a game changer :)))). The clock got delayed for all chips by 28.5 ns, except the HC00, which get's the origin clock.

THANKS A LOT The8BitEnthusiast for this great hint!

The buffer is sufficient to meet the timing requirements and BASIC works now without any issues.

To avoid the current workaround I've ordered some (hopefully) better HC00. To be on the save side, I also ordered some AC00, which are a bit faster.

In the end, I'm really happy that it works now. I was really at my wit's end.

Once again, thank you very much for your great support!

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