_drupal_exception_handler Error: Uncaught Error: Call to undefined function cache_get()

After upgrading some D7 sites to a new dedicated server, I am getting exatly the same issue.

PHP version 7.3.31-1~deb10u6, the sites are on Drupal 7.100 respectively 7.99 in one case.

Drush cc:

$ drush cc
Enter a number to choose which cache to clear.
 [0]  :  Cancel   
 [1]  :  drush    
 [2]  :  registry
2
PHP Fatal error:  Uncaught Error: Call to undefined function cache_get() in /var/www/{mysite}/includes/module.inc:763
Stack trace:
#0 /var/www/{mysite}/includes/module.inc(963): module_implements('system_theme_in...')
#1 /var/www/{mysite}/modules/system/system.module(2531): module_invoke_all('system_theme_in...')
#2 /var/www/{mysite}/includes/theme.inc(798): _system_rebuild_theme_data()
#3 /var/www/{mysite}/includes/theme.maintenance.inc(57): list_themes()
#4 /var/www/{mysite}/includes/bootstrap.inc(2965): _drupal_maintenance_theme()
#5 /var/www/{mysite}/includes/errors.inc(177): drupal_maintenance_theme()
#6 /var/www/{mysite}/includes/bootstrap.inc(2690): _drupal_log_error(Array, true)
#7 [internal function]: _drupal_exception_handler(Object(Error))
#8 {main}
  thrown in /var/www/{mysite}/includes/module.inc on line 763
Drush command terminated abnormally due to an unrecoverable error.                                           [error]
Error: Uncaught Error: Call to undefined function cache_get() in /var/www/{mysite}/includes/module.inc:763
Stack trace:
#0 /var/www/{mysite}/includes/module.inc(963): module_implements('system_theme_in...')
#1 /var/www/{mysite}/modules/system/system.module(2531): module_invoke_all('system_theme_in...')
#2 /var/www/{mysite}/includes/theme.inc(798): _system_rebuild_theme_data()
#3 /var/www/{mysite}/includes/theme.maintenance.inc(57): list_themes()
#4 /var/www/{mysite}/includes/bootstrap.inc(2965): _drupal_maintenance_theme()
#5 /var/www/{mysite}/includes/errors.inc(177): drupal_maintenance_theme()
#6 /var/www/{mysite}/includes/bootstrap.inc(2690): _drupal_log_error(Array, true)
#7 [internal function]: _drupal_exception_handler(Object(Error))
#8 {main}
  thrown in /var/www/{mysite}/includes/module.inc, line 763

Result is: sites are inaccessible, getting an "Error 503: Backend unavailable" from the SSl gateway.

No solution available five years after issue was reported. Changing priority to critical.