Can you copy & paste the error message that appeared on the plugins page?
Internal Server Error.
The server encountered an internal error or misconfiguration and was unable to complete your request. Please contact the server administrator and inform them of the time the error occurred, and the actions you performed just before this error. More information about this error may be available in the server error log.
When I then attempt to click on any other WP menu item afterwards, I get a blank page with the following sentence:
Briefly unavailable for scheduled maintenance. Check back in a minute.
It sounds like the WordPress plugin updater experienced a glitch. Do you see a .maintenance file in your site root folder (same folder as wp-config.php)? If so, removing that should take your site out of maintenance mode.
Yes, removing the .maintenance file from the root folder fixes the maintenance issue. However, trying to update the plugin afterwards results in the same outcome as described above: Server Error. Followed by the maintenance message again.
