Are you using UME exports? "Classic" QuickTime doesn't work with Catalina.

Is the Avid at the other end licensed to run virtually? Are you logged in as the same user with the same privlidges when both local and remote?

Apple doesn't want you putting anything on the root (top most) level of the boot drive. Avid requires the Avid MediaFiles folder must be on the root level. Avid also "strongly discourages" putting your media on your boot drive. So...outside of that... If you can't/won't/don't want to use an external drive, you could try to

Avid changed the names on the DNxHD codecs when they became resolution independent. DNX LB in HD is DNX 36 with a different name. Unfortunately, computer programs don't do well with nicknames so you need a updated codec package to understand that they're the same. QuickTime Player X will read updated codecs from Apple's ProVideo Formats

And the raster size of the master clip is 4x3 or is it 16x9 format?

Media Composer might be using the setting from the master clip that the sub-clip came from rather than a sub-clip that was used to make another sub-clip. What's the re-format setting of the master clip that the sub-clip is coming from?

When in segment mode (red arrow) you can map a key to move clips up and down tracks but the only option to move a selected clip is using the trim adjustment keys (m , . / also known as << < > >>). If you want to move a segment by a certain amount, use trim mode.

Last time I used one the fan was loud and it used SoftRAID rather than a hardware RAID. Outside of that, it performed well.

Try changing the permissions on the Avid Media Composer/Settings folder. Select the folder, hit Cmd-I to open the Get Info box and at the bottom of the window under Sharing & Permissions, change "everyone" to read/write access. (You'll have to click on the lock icon in the lower right hand corner and put in the admin password.). Before

You might try using a new user settings file to see if it behaves more predictably. Using a user settings file from a older version sometimes causes "unexpected consequences". Generally this happens between major releases but I had a weird problem with Workspaces not behaving as expected when going from 2021.5 to 2021.6.