Hi @Gayle Kremer ,
Do you deploy Office Online server to open Excel in SharePoint 2013?
If it is true, please try to set Excel timeout in Office online server as following:
Add the following value to the settings.xml file, where TimeoutValue is the timeout value in milliseconds. (This file is normally located at C:\ProgramData\Microsoft\OfficeWebApps\Data\FarmState\settings.xml.)
<Setting Name="FBDirectReadTimeoutInMilliseconds" DataType="System.Int32">
<StringValue>TimeoutValue</StringValue>
</Setting>
If it is not feasible for you, please feel free to reply.
Similar issue:
Reference:
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