Is this when opening an existing event ? Did anything change from the last time you opened it ?
Or are you creating a new event ?
Sometimes a parsing error like this can be caused by a faulty user input, in a meta field for example.
Are you using a custom control/widget ?
You seem to be using Sitefinity v.9, did you upgrade your project or created it in v.9 ?
If it was upgraded, did it run successfully after that ? What changed that you suddenly get this error now ?
Are you getting any other errors?
What you can do is first delete the files under ~/App_Data/Sitefinity/Logs and reproduce the issue. This way only the error messages related to the issue will be logged in the files and you could attach the logs to this thread as they may have some more information.
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