When starting ArcGIS Pro version 2.6.x and earlier, an ArcGISCleanup.exe - Bad Image warning occurs:
ArcGISCleanup.exe - Bad Image <path to a .dll file> is either not designed to run on Windows or it contains an error. Try installing the program again using the original installation media or contact your system administrator or the software vendor for support.
This warning is thrown by Windows when an attempt is made to load a 32-bit .dll into a 64-bit process, such as starting ArcGIS Pro. This is an illegal operation to the operating system.
Certain software packages install on a machine such that their components (DLLs) are automatically loaded into every process that starts on the machine. This is particularly common with anti-malware software. However, if a 32-bit DLL is loaded with a 64-bit process, the operating system throws a warning.
This warning may be suppressed in some 64-bit programs. ArcGISPro.exe, for example, suppresses the warning to the end user, but ArcGISCleanup.exe does not. Any 64-bit program installed on the machine that does not actively suppress these warnings will also display this warning.
The warning does not appear at ArcGIS Pro version 2.7 and later. In earlier versions of ArcGIS Pro, the warning can be safely ignored.
To remove the warning entirely, the 32-bit program can be uninstalled or updated from the software vendor (in the case that a newer version of the software has a fix). The issue can also be reported directly to the software vendor.