In both Windows 10 and Windows 11, it may happen that File Explorer freezes or appears to freeze when you try to mount an ISO file.
The solution is to temporarily disable a setting and re-enable it immediately after mounting. The setting in question is:
Start(menu) → All → Windows Security → App & browser control → Reputation-based protection → Check apps and files
The reason this setting can cause File Explorer (or more precisely, explorer.exe) to hang is that ISO files are often quite large. Scanning these files can therefore cause Windows to appear unresponsive. To be more precise: it usually freezes the specific Explorer window involved.
Of course, we look forward to a permanent fix from Microsoft, especially since this issue has been present since the very first version of Windows 10, as far as we know. Until then, the workaround above can be used. It even works if you are already in the middle of mounting an ISO that currently seems stuck—the effect is immediate.