Q: imac crashes after installing  flash

Did you try the Adobe Flash Player uninstaller that usually is included with

a Player download, to see if removing the player & a restart, stops crashes?

• Try safe mode if your Mac doesn't finish starting up - Apple Support

The Console logs will show a major list of anything going on in the computer

and to look into that based on time day (hour min sec) you should be able to

locate logs from the time of these crash events. The causes of kernel panic

and other crash can be based in the complex and sometimes incompatibility

of different versions of software that may not be designed to work in OS X

after an upgrade to a later version newer system.

Also, add-ons, extensions, other items that you may have installed on purpose

or by accident, may be part of the cause of system issues and crash events.

Did you get the Adobe Flash player from their official web site? Posers that

offer or tell you to get Flash player from a popup in your web browser may

be tainted with adware, malware, or not really be what they suggest they are.

• Flash Player Help - Adobe - Mac:

https://helpx.adobe.com/flash-player/kb/installation-problems-flash-player-mac.h tml

There is no Flash player installed in my Mac computers, as I'm done with it.

Even the older ones here with ancient Leopard 10.5.8 don't use it; the better

browsers can use HTML5 for animations, as primary means.

However the Console logs should reveal cause of the time based crash or

kernel panic, etc:   • Mac OS X: How to log a kernel panic - Apple Support

With a copy of the crash log, you could post it in an ASC discussion and hope

one of the expert contributors here can help diagnose or suggest options.

Good luck &  happy computing!

DId you get the real Adobe Flash Player, or from another nefarious source masquerading as an official Adobe Flash update panel in your browser?

Both Safari, and the System Preferences panels must be quit before launching the real Flash installer.

Did you install any other applications, browser extensions, etc. in addition to Flash Player? I ask this, because in the years that I have been using the Adobe Flash Player, it has never crashed my Macs on any release of OS X.

Try un-installing and then re-installing.

Adobe Flash Uninstaller

Adobe Flash Player