If you search “A serious error occurred please contact the administrator“, you will see many posts. There are more than one scenarios of this error. I have tried to consolidate most obvious cases of this error.
Scenario 1 (I was facing this issue): Social Media is disabled in your configuration file but related items do exist in CORE database.
Error in log file: Error processing command url:/-/speak/request/v1/expeditor/ExperienceEditor.Social.SocialCenter.GetMessagesCount error:System.InvalidOperationException: Could not retrieve request class for url:/-/speak/request/v1/expeditor/ExperienceEditor.Social.SocialCenter.GetMessagesCount at Sitecore.ExperienceEditor.Speak.Server.RequestHandler.Process(HttpContext context) at Sitecore.ExperienceEditor.Speak.Server.RequestHandler.ProcessRequest(HttpContext context)
Solution: Remove these two items from CORE DB:
Scenario 2: URL Rewrite issue with SPEAK request
Solution: Well explained in given blog (However, this is not the problem I was facing).
Scenario 3: Issue while switching between Sheer UI and SPEAK modes in Experience Editor.