The server encountered an internal error or misconfiguration.”
This is usually caused by VPS hitting its memory parameters, the QOS alerts can be checked from vzmc/pmc to confirm this.
A temporary solution would be restart the the vps to release the memory resource. You want to fix it permanent solution make sure to check how frequent the QOS parameters are hitting their limit and what is causing this high usage and resolve that accordingly.
After restarting virtual private servers , make sure to check it in new browser window about the Plesk login information.