cannot open disk image due to invalid argument

1. I suggest to do a SMC reset, and a PRAM reset. Not absolutely necessary but it won't hurt either.

Reset the System Management Controller (SMC) on your Mac - Apple Support

How to Reset NVRAM on your Mac - Apple Support

2. If that does not help:

Start in Safe Mode:

You have to have an account password to logn the SafeMode, if you do not have one, create one first in SystemPreferences->Users&Groups->Password tab

Hold the Shift key and start up the mac, hold the shift key till about halfway the progress bar, it is sloooow, login with your password. Then click the Apple icon and select restart.

3. If that does not work, we have to reset all user permissions, that is a bit more complicated. Tell me how it goes.

Can you mount the disk image while booted into Safe Mode?

Reinstall OSX from the Recovery Partition.

Welcome. It seems so far anyway that's the most successful way of fixing .dmg issues based on other posts.

Could be that dmg is somehow corrupt

Get it again, using

https://www.adobe.com/products/flashplayer/distribution3.html

Do you get the same message when trying to open any saved dmgs? What happens if you try mounting it from Terminal?

hdiutil attach /path/to/diskimage.dmg

If you're not afraid of running some commands in Terminal, and you have a known, good working backup, where Disk Images mount OK, then please post back. It would involve moving over a good DiskImageMounter.app in /System/Library/CoreServices. The one you are currently using might be corrupt.

And did you try mounting it from the command line?

hdiutil attach /path/to/diskimage.dmg

If the dmg is on your Desktop, it would be (if it's the latest Flash)

hdiutil attach ~/Desktop/install_flash_player_21_osx.dmg

I was unavailable.

When the possibility of a coreservices corruption (WZZ) is there this will not help:

We use the Reset Password Utility in the Recovery Partition in ElCapitan to reset user permissions:

1. Restart your computer from the recovery partition ((start while holding the CMD+R keys).
2. Open Terminal from the Utilities menu.
3. Type in resetpassword , a window called „select the volume containing the user account“ will be opened, select the startup disk
4. do nothing further, but click the Reset button at the bottom right side of the window
5. Now Quit the Password Utility from its menu, going back to the main Recovery Partition screen.
6. On your keyboard, hit ⌘ + Q and restart your computer. It's very important that you don't hold down the power button to exit the recovery session, or the ACL reset won't occur.

Resetting User Permissions will not have any effect on an Apple installed item, such as anything in /System/Library. And Permissions repair for Apple installed items is gone in El Cap with SIP.

Strictly limited to your Home Folder/User.

Right, that is why I said:

When the possibility of a coreservices corruption (WZZ) is there, this will not help

Lex