What does the popup mean?...
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[icon. white exclamation point inside a golden triangle]
The system extension
"SyminternetSecurity.kest" is not
compatible with this version of OS X and
can't be used.
Please contact the developer for updated software.
[ OK ]
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iMac, macOS Sierra (10.12)
It means you have some third-party security crapware installed that is not compatible with Sierra. I'm guessing it is Symantec or Norton based on the "Sym" in front of the name.
Just use their Uninstaller to remove the entire program. The only thing it ever does is cause problems like these.
Uninstall third party apps that claim to protect, clean, boost performance, manage, etc. your computer. They are not needed and may cause problems. Symantec Norton Anti Virus is an example of one such app.
SyminternetSecurity.kest
