Hello Axel,

According to your description, you got an error message reads "there is no
build provider registered for the extension" when opening aspx pages in
source view. If I misunderstood anything here, please don't hesitate to
correct me.

This line of error message means VS IDE cannot found build provider
registered for .aspx extension.

Does this issue occur on each web project on your machine?

1)    If it only occurs on your current project, please check your project
web.config file.
Did you add the entry as below in your config file? If so, please remove it.
<buildProviders>
<clear/>
</buildProviders>

2)    Otherwise, please check your web.config in the .net framewok folder
(C:\Windows\Microsoft.NET\Framework\[version]\CONFIG) if the issue occurs
on each web project on your side.
Is the following entry located in your config file? if not, please add it.
<buildProviders>
<add extension=".aspx" type="System.Web.Compilation.PageBuildProvider"/>

3)    At last, if the issue still persists, please register build provider in
your project web.config file directly.
<buildProviders>
 <add extension=".aspx" type="System.Web.Compilation.PageBuildProvider"/>

Hope this helps. Please try the above method and let me know if this works.
We are glad to assist you.

Have a great day,
Best regards,

Wen Yuan
Microsoft Online Community Support
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