I guess your licensing code is not processed properly so the API will be used in evaluation mode. Kindly make sure that your licensing code should be processed first before using any other Aspose.Cells APIs in your code. Please see the document for your reference:
Generally, the license should be set on the beginning of the code, before any other functionality of Aspose.Cells is used. The license should be set only once, and it will remain licensed until the application is closed. In case of ASP.NET (IIS based) web application, good place to set license is Application_Start method. On this way you are sure that the license is set when application is started, and the license is set again if application is recycled for any reason.
One possible reason for the issue could be if you use the product after the web application is recycled and before the license is set again.
Also, you may evaluate (at any place) if the license is set or not, see the document for your reference:
Let us know if you still have the issue.