First, the obvious

An 877 with a 20MHz crystal does work

Yours doesn't. So -

Likely candidates are the crystal or the 877. There is the remote
possibility that the PCB track(s), for some reason, are not 100%
and are being coincidentally repaired / broken as you swap crystals

The crystal could be checked with a couple of TTL gates. If it
checks out OK, then that leaves the 877

So what could go wrong there

Fuse settings, below-spec part. Not a whole lot of options really.
The 877 has a fairly basic oscillator section and can't say I've ever
had any problem like yours

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