Are you seeing a specific error? Did you turn "error reporting" to maximum in Joomla's Global Configuration?

Some users have reported that such issues usually occur from third-party extensions that interface with K2.

Have a look in the plugins for anything related and disable one by one to see if that resolves your issue.

Not at all. K2 works just fine from PHP 5.6 to 7.3 (and soon with 7.4 on K2 v2.10.3).

Please do take the time to check for any additional plugins. If you did that already, perhaps your local Apache server has issues processing the image upload request. In which case you'll have to find Apache's error logs and look for any entries with "com_k2" included.