Same error for me, coming from Windows 11 Pro RP 22621.169. Impossible to install KB5015818. Always failing with error 0x800f0911.
CBS.log file is showing:
Error CBS Modified marker file present in source package folder: \?\C:\WINDOWS\SoftwareDistribution\Download\6adaacacb4f20fc12c73d96fab81cfb4\metadata\Windows11.0-KB5015818-x64\ [HRESULT = 0x800f0911 - CBS_E_SOURCE_MODIFIED]