Calendar App Glitching Upon Open

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Next, log into a new, basic, admin user account, Set up users, guests, and groups on Mac,  and see if the problem persists there. This tells us if the problem is limited to your user account or is system wide.

If the problem does not exist in the other user account it means that there's something in your account that's causing the problem. Go back to your account, download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

What you might do is go into the other user account and locate the Calendar folder in the Home/Library folder. Copy it to the Users/Shared folder.

Then go back to your primary account and move the same Mail folder to the Desktop. Place the Mail folder from the Shared folder in the Library and reboot. Then check the calendar to see if it's settled down.

NOTE: make sure that the calendars in the other user account are accurate and complete before proceeding.

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Also, unless you're using a true VPN tunnel, such as between you and your employer, school or bank's servers, they are useless from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Former Malware Distributor Kape Technologies Now Owns ExpressVPN, CyberGhost, Private Internet Access, Zenmate, and a Collection of VPN “Review” Websites. 

That being said you've instead what many here consider to be akin to malware: CleanMyMac.

Uninstall the following in accordance with the developer's instructions:

CleanMyMac

CleanMyDrive

Gemini

PACE Anti-Piracy

Nord VPN

Pulse Secure.

If your employer requires a VPN use only the one they provide. The other apps are will only be resource hogs and interfere with proper system and application performance.

Good to hear. 👍