ERROR: CheckIndexAgainstCheckpoint: forked chain older than last checkpoint (height 504032)

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### Describe the issue

### Can you reliably reproduce the issue?
#### If so, please list the steps to reproduce below:
1.
2.
3.

### Expected behaviour
Tell us what should happen

### Actual behaviour
Tell us what happens instead

### Screenshots.
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### What version of bitcoin-abc are you using?
List the version number/commit ID, and if it is an official binary, self compiled or a distribution package such as PPA.

### Machine specs:
- OS:
- CPU:
- RAM:
- Disk size:
- Disk Type (HD/SDD):

### Any extra information that might be useful in the debugging process.
This is normally the contents of a `debug.log` or `config.log` file. Raw text or a link to a pastebin type site are preferred.

Start at the CheckIndexAgainstCheckpoint error and gather the missing reproduction steps, version or commit, machine details, and debug.log requested by the issue template. Compare the observed chain and checkpoint state to establish a reproducible failure and document the cause and verification criteria.

Written by the indexing model from the issue text.