Can't select files in file selection windows

Check to see if the problem occurs:

a) In Safe Mode;

b) booted normally, and logged in to a new user account.

Tell us, and also Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

Adware:

Launchd: ~/Library/LaunchAgents/com.spigot.ApplicationManager.plist

Executable: ~/Library/Application Support/Spigot/ApplicationManager --protect

Reason: Adware name match

Should “ Certain & Specific Software “ referenced from above be installed - it will negatively impact macOS. It is suggested, to download directly from a respected ASC Contributor, the application Malwarebytes for Mac. 

It is free or paid for added features. 

Disclaimer: I am not affiliated with nor receive compensation from the Developer.

Run the Application and it should remove the malware / adware. Once done, restart computer and test.

DrCleaner.app Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

This includes AntiVirus, Disk Cleaners, Disk Optimizes, UnInstaller etc.

System modifications - There are a large number of system modifications running in the background.

Launchd: ~/Library/LaunchAgents/com.bittorrent.BitTorrent.plist

Executable: /Applications/BitTorrent.app IMHO a possibly Dangerous Source of Adware and Malware

Looking at the Supplied Report - do not suggest this radical step too often. Because here are so many Modifications and Extensions installed - to clean it up would be a Major Undertaking to put things right.

Suggestion and Personally - would wipe the drive and reinstall the OS. Then reinstall third party applications that are absolutely Required to function. Plus, would Not use Time Machine Backup to Restore anything as this would just restore the same issues again.

Unsigned files - There are unsigned software files installed that could be adware and should be reviewed.

Configuration Files: /etc/hosts - Count: 16

System modifications - There are a large number of system modifications running in the background.

x86-only Apps - This computer has x86-only apps might not work on future versions of the operating system.

Kernel extensions present - This computer has kernel extensions that may not work in the future.

System Extensions: there are so many Extensions installed and they may a source of issues

This system seems to have been upgraded in place for many years - there is an install from 2013.

You have so many system modifications, several of them a few years old.

At this point, I do believe that a clean install is recommended.

If you don't feel up to it, try installing a clean system on an external SSD and see how the system behaves. I expect it will work all right.

Welcome and thank you too.