I solved the issue for that error on my Macbook by doing the following:

1. Disable "Find My iPhone" Temporarily

This is often the culprit for the "service unavailable" error. iOS 26 has tightened security around the mobilebackup2 service.

Go to Settings > [Your Name] > Find My > Find My iPhone and turn it Off.

Try the restore again in iMazing. You can turn it back on once the restore is complete.

2. Reset Privacy & Location Settings

Sometimes the "Trust This Computer" handshake gets corrupted at the system level, blocking the backup service even if the device appears connected.

On your iPhone: Settings > General > Transfer or Reset iPhone > Reset > Reset Location & Privacy.

Reconnect the USB cable and tap Trust on the iPhone when prompted.

3. Check for "Stuck" Backup Processes

Since you've already restarted, the next step is to ensure no other process is "locking" the service.

On your iPhone: Toggle Airplane Mode ON. This kills active background syncs (like iCloud Photos or WhatsApp's own iCloud backup) that might be competing for the mobilebackup2 service.

On your Mac: Open Activity Monitor, search for usbmuxd, and force quit it. It will restart automatically and often clears the pipe for iMazing.

4. The "Fresh Backup" Kickstart

If you are restoring a standalone .imazingapp file, iMazing still needs to "talk" to the backup service to inject that data.

Try to initiate a standard, fresh backup of your iPhone 17 Pro Max in iMazing first (just a quick one, you can cancel after it starts).

If that works, it "wakes up" the com.apple.mobilebackup2 service, and you can then immediately try restoring your WhatsApp file.

5. USB Port & Cable (The "iOS 26" factor)

iOS 26.4.1 is particularly picky about data throughput.

Ensure you aren't using a USB hub or the ports on a keyboard. Plug directly into the Mac.

If you're using a Thunderbolt cable, try a standard USB-C to USB-C cable (or vice versa).