This can happen when you’re running sites behind something like Cloudflare. But also happens if you are on a network where your IP address can change.
Here’s instructions for the config option you can create to make Concrete not log users out on IP address change:
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If you are using Cloudflare though, it’s better to not adjust that setting, and instead set up the trusted proxies. In that case you’d search for ‘Trusted Proxes’ in the dashboard, paste in the two sets of IP addresses here: IP Ranges
and check the headers to be forwarded (I believe you’d check all except the last one)