Since your site is currently a subdomain, there are three common reasons why these runtime/server errors occur. Once your site is connected to your domain, these details will no longer be relevant.
1) You need to clear your cookies, cache and browsing history. I’ve provided the how to article below:
2) The page has timed out. It typically takes an hour for a page to time out. If you're in the middle of setting up your site and you happen to leave the page open for an hour or longer, the page will expire and all unsaved work will be lost. You simply need to refresh the page and it will default reload to the landing page.
3) You attempted to open multiple tabs/windows of your site on the same web browser or you attempted to login as different users in the same web browsing session. If you want to login as different users at the same time, you can do so by opening up an incognito or private window and logging in as one user there. It is not possible to open multiple tabs/windows of your site in the same browsing session nor is it possible to sign in as different users in the same browsing session, unless you use an incognito or private window.
If the server errors continue, please let us know and we will have our development team take a deeper look.