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Connection Closed Error and Attempting to Reconnect


Product Editor for Blazor,
FileManager for Blazor,
FileSelect for Blazor,
PdfViewer for Blazor


This article is relevant to different scenarios and Telerik Blazor components. Here are a few examples:


  • Pasted images inside the Editor are discarded. They are not converted to base64 and not saved to the database.
  • Pasting large images or large data always drops the Blazor app SignalR connection. The browser console shows "Attempting to reconnect" message and I have to reload the page.
  • Large HTML content in the Editor restarts the Blazor app connection.
  • The Editor freezes when copying and pasting long text.
  • An error shows when the Editor contains a large document and the HTML string has lot of characters.


  • The FileSelect OnSelect handler fails on Stream processing.
  • FileSelect upload scenarios break with large files. The Stream has no data.
  • Cannot CopyToAsync the FileSelect Stream to a new MemoryStream. There is no error message.
  • Files are written to the server directory with a size of zero bytes.
  • The FileSelectFileInfo.Stream.CopyToAsync method hangs. Execution stops and no exception is thrown. The next line in the code is never reached. I am attempting to load a JPG image into the MemoryStream.


The PDF Viewer doesn't display PDF files, which are assigned to its Data parameter, but it displays local files from the user device.

Import XSLX Files

Cannot import XSLX files which have a lot of records. It works for few records. It's a random behavior.

Error Message

The exceptions may be similar to:

Connection disconnected with error 'Error: Server returned an error on close: Connection closed with an error.'

Attempting to reconnect to the server...



A task was canceled.

Possible Cause

Blazor Server apps use a SignalR WebSocket to communicate between the client (browser) and server. The SignalR WebSocket has a default maximum message size of 32 KB. A large Editor Value, FileSelect file size, or a PDFViewer Data can exceed the maximum SignalR message size, which will close the connection and abort the current application task.


Increase the maximum SignalR WebSocket message size (MaximumReceiveMessageSize) for the Blazor application in HubOptions.


Make sure that AddServerSideBlazor() is called only once. All configuration settings in AddHubOptions and AddCircuitOptions must go together with this single statement.

See Also

In this article