After installing the September 2026 updates and rebooting, the SolidCP Server endpoint returned Server Error in '/' Application – Runtime Error for every request. The Enterprise Server could no longer reach it, so all panel operations against that server failed — creating or disabling users, mailbox actions, everything.

our workaround: Put Newtonsoft.Json 13.0.0.0 into SolidCP's bin. If Exchange is on the same box, it ships a copy.

Copy-Item 'C:\Program Files\Microsoft\Exchange Server\V15\bin\Newtonsoft.Json.dll' `
          'C:\SolidCP\Server asp.net v4.5\bin\Newtonsoft.Json.dll' -Force

Add a binding redirect inside the existing <assemblyBinding> block in Web.config (the one that already contains <probing privatePath="..."/> — do not create a second block):

<dependentAssembly>
  <assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral" />
  <bindingRedirect oldVersion="0.0.0.0-13.0.0.0" newVersion="13.0.0.0" />
</dependentAssembly>

The redirect matters: SolidCP.Providers.DNS.SimpleDNS80.dll references Newtonsoft.Json 10.0.0.0, so without it that provider would break instead.

Maybe this helps someone...