Active Users cannot login

[CarolSEL]

Random active users are unable to successfully login. Attempts return a message saying they are not logged in or don't have permission to access that page. Using different browsers sometimes helps, but while some can only log in from Chrome (or whatever), others using the same browser cannot. Devices can be anything from phone to PC. Nothing in their profile seems to be in conflict. Clearing cache doesn't solve the problem. No error messages are reported from the server.

We're getting a fairly large influx of new members, but are not seeing them online. And long-time members find they suddenly cannot access us. We've lost some valuable members because of this issue.

Any ideas of where to look for a problem?