Photos stopped playing videos and projects

The fact that the problem doesn't exist in Safe Mode tell us you have 3rd party software that is causing the problem.

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.  

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint.  Read these two articles: Public VPN's are anything but private, Former Malware Distributor Kape Technologies Now Owns a Number of Public VPNs and a Collection of VPN “Review” Websites and Security Risks: The Dangers of Using Free VPNs (eccu.edu)

It's my recommendation that you should uninstall the anti-virus and VPN software according to the developers' instructions.

There are a couple of things you can try:

1 - Boot into the Recovery volume (boot with the Command + R keys held down  - Intel Macs) or  (How to Boot an M1 Mac into Recovery Mode), select Disk Utility and repair all available containers/volumes on the boot drive.

If necessary continue with:

2 - 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.

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

You might try creating another user account (if you can) without the VPN or A/V activated and run Photos from there.

OR, use another video player like VLC, MPlayer or Cistern Video Player.