Is it no longer possible to execute "Publish" from the Detail page of a Customization Project, starting with version 26R1?

Hello ​@AndrewZ You can follow this points:
 

Acumatica Support confirmed this fix for .NET Framework 4.8+ compatibility issues:​

1. Open web.config in your Acumatica site root

2. In the <appSettings> section, add:

   <add key="aspnet:DisableAppPathModifier" value="false" />

3. Save the file and retry publishing from the editor

Stale validation files can cause hash mismatches:​

1. Navigate to your Acumatica site's C:\Windows\TEMP\ and delete old 7764f32b-... folders

2. Also clear the Customization\[SiteName]Validation\ folder inside your Acumatica instance directory

3. Restart IIS and retry

Unpublish All, then Republish.

I hope this helps.