Pro Sites giving frontend PHP output errors after server upgrade

We have our site anchorgiving.com hosted on SiteGround, which did some nonspecific "server upgrades" early this morning. Our site was working perfectly as of last night. This morning after the server upgrades, our site started having the following PHP output errors on the site:

Warning: session_start(): open(/tmp/sess_saqqk6vr5p751thjb4dgq50bt6, O_RDWR) failed: Permission denied (13) in /home/giveloca/public_html/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php on line 29

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/giveloca/public_html/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php:29) in /home/giveloca/public_html/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php on line 29

We've done some troubleshooting (disabled all other plugins, tried a default WP theme) and have determined it's an issue with the Pro Sites plugin. But it was working perfectly as of last night and we've made no other changes.

This is ONLY happening on Chrome OSx and Windows. We aren't getting the errors on Firefox or Safari or iOS. Furthermore, the error doesn't appear on Chrome incognito mode. It's pretty weird.

Any idea what could be causing this? We've gone ahead and lodged a support ticket with SiteGround, too, but hoped someone on this end could shed some insight into this.

Edited to add: SiteGround says all they did was move the server to another hardware node and that this shouldn't have changed anything about the configuration of the server or broken the plugin.