Thank you for the replay, @caseproof!
We have been running the Members plugin on all four of our websites for quite some time with no problems, so I know it isn’t a setup issue. The errors only occurred on one of our websites after we upgraded the PHP on that site to version 8.1.11, which is why I thought it might be a PHP issue.
For now, I have turned the plugin off until I can identify the cause of the problem–though I would love to turn it on again sometime in the future. If you have any other suggestions at some point, let me know.
Thanks!