The common causes for a session mismatch error are usually one of the following:
1. Mis-configured URL parameter in your config.json file
If you have a misconfigured url value in your config.json file, the cookie may be saved incorrectly (or not at all), causing a session mismatch error. Please ensure that the link you are accessing your site with and the url defined match.
2. Improper/malformed cookieDomain set in ACP
Sometimes admins set this value without realising that they probably don't need to set it at all. The default is perfectly fine. This is what the config looks like:
Cookie Domain setting
If this is set, you'll want to revert the setting by editing your database directly: