Quote from: marc3535 on January 27, 2024, 10:07:45 AM

This is a fragment of the debug.log file after restarting Bitcoin Core; does it contain any useful information?

Not much since the log before the restart isn't included for comparison.
But there are indications that the datadir that it loaded doesn't have the full blockchain,
since it's after the restart from a allegedly fully synced blockchain, it shouldn't be like than unless it's an issue with your Mac or a change in the previous setting.
Plus the date doesn't match, it looks like the logs after following the first few replies. (I see it in the config)

And there are some inconsistencies like this:

Quote from: marc3535 on January 27, 2024, 10:07:45 AM

2024-01-26T13:49:59Z Config file arg: datadir="/Volumes/ext SSD/blocks"

That is the blocks directory and should be set as "blocksdir=" if you do not want to save the blocks inside the datadir.
Since you've set it like that, it would use "/Volumes/ext SSD/blocks" as the datadir and save the blockchain in "/Volumes/ext SSD/blocks/blocks".
Yet your node shows that it's using "/Volumes/ext SSD" as the datadir since you've set it in the GUI's welcome screen.
(assuming that there are no lines omitted in the gap in the middle of the shared logs)
Just remove the datadir setting in the bitcoin.conf file since it's incorrect and does nothing.

Quote from: marc3535 on January 27, 2024, 09:40:48 AM

I understand what you say. But the debug.log file contains thousands and thousands of lines of information. I really wouldn't what to search for there...

You can paste it inside the code tags like I've instructed in my earlier reply.
That way, it wont consume a lot of vertical space.

You can also pick the sessions that are relevant like you did in the first log that you shared.

Quote from: marc3535 on January 27, 2024, 06:50:09 PM

Thanks. This is the output of getblockchaininfo. Can you see if it has reset the block download progress? Or maybe just again syncing?

  "blocks": 535936,
  "headers": 826689,
  "initialblockdownload": true,

It's syncing and currently at 535936 height.