This error is indicating that the file does not exist or that there are no write permissions to this file.
In this case, there are no write permissions only at the exact instant the websave is called in the parfor loop and not otherwise.
This is because the file is being saved and deleted from a Dropbox folder. If the Dropbox folder is connected to the internet, it will constantly be syncing. This can cause delays in when files are saved and deleted and can therefore cause write issues.
Save the files to a local folder or turn off the Dropbox connection to the internet so it does not automatically sync.
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