Yolo can you verify the website can only be accessed via the canonical URL? Maybe those users accidentally ended up on the non-https or wrong www subdomain by typing wrong address or following wrong internal links.
If that doesn't help, it might be helpful to ask each of those users which browser and operating system they experienced the issue on to try to figure out if it only happens on specific systems.
If it's completely random, you could try to disable extensions to see if anything changes, but you would need a way to reliably reproduce the issue to not loose too much time troubleshooting this part.
