There is a related issue with pretty much the exact same problem, but it was last active a year ago, so I figured I open a new one. The files sharing API seems to be broken in Nextcloud Version 28 and 29 and throws CORS errors. Authorized file access is not affected and works as intended.
The files sharing API works fine in my local 26 version and it used to work in our production environment until we upgraded to v28. I'm not 100% sure however if our live Nextcloud was upgraded over time or all at once to v28 at some point, so there's a slight chance the breaking change might have been introduced in v27 instead.
{
"reqId":"IX66HIjeyKVHhi4y9ff7",
"level":3,"time":"2024-06-12T09:45:25+00:00",
"remoteAddr":"172.17.0.1",
"user":"--",
"app":"index",
"method":"OPTIONS",
"url":"/index.php/apps/webapppassword/api/v1/shares",
"message":"Cannot assign null to property OCA\\Files_Sharing\\Controller\\ShareAPIController::$currentUser of type string",
"userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/125.0.0.0 Safari/537.36",
"version":"29.0.1.1",
"exception":{"Exception":"TypeError","Message":"Cannot assign null to property OCA\\Files_Sharing\\Controller\\ShareAPIController::$currentUser of type string","Code":0,"Trace":[{"function":"__construct","class":"OCA\\Files_Sharing\\Controller\\ShareAPIController","type":"->","args":["webapppassword",["OC\\AppFramework\\Http\\Request"],["OC\\Share20\\Manager"],["OC\\Group\\Manager"],["OC\\User\\Manager"],["OC\\Files\\Node\\LazyRoot"],["OC\\URLGenerator",["OC\\User\\Session"]],["OC\\L10N\\LazyL10N"],["OC\\AllConfig"],["OC\\App\\AppManager"],["OC\\AppFramework\\DependencyInjection\\DIContainer"],["OC\\UserStatus\\Manager"],["OC\\PreviewManager"],["OC\\DateTimeZone"],["OC\\AppFramework\\ScopedPsrLogger"],null]},{"file":"/var/www/html/lib/private/AppFramework/Utility/SimpleContainer.php","line":83,"function":"newInstanceArgs","class":"ReflectionClass","type":"->","args":[["webapppassword",["OC\\AppFramework\\Http\\Request"],["OC\\Share20\\Manager"],["OC\\Group\\Manager"],["OC\\User\\Manager"],"And 11 more entries, set log level to debug to see all entries"]]},{"file":"/var/www/html/lib/private/AppFramework/Utility/SimpleContainer.php","line":128,"function":"buildClass","class":"OC\\AppFramework\\Utility\\SimpleContainer","type":"->","args":[["ReflectionClass","OCA\\WebAppPassword\\Controller\\ShareAPIController"]]},{"file":"/var/www/html/lib/private/AppFramework/Utility/SimpleContainer.php","line":146,"function":"resolve","class":"OC\\AppFramework\\Utility\\SimpleContainer","type":"->","args":["OCA\\WebAppPassword\\Controller\\ShareAPIController"]},{"file":"/var/www/html/lib/private/AppFramework/DependencyInjection/DIContainer.php","line":470,"function":"query","class":"OC\\AppFramework\\Utility\\SimpleContainer","type":"->","args":["OCA\\WebAppPassword\\Controller\\ShareAPIController"]},{"file":"/var/www/html/lib/private/AppFramework/DependencyInjection/DIContainer.php","line":442,"function":"queryNoFallback","class":"OC\\AppFramework\\DependencyInjection\\DIContainer","type":"->","args":["OCA\\WebAppPassword\\Controller\\ShareAPIController"]},{"file":"/var/www/html/lib/private/AppFramework/App.php","line":163,"function":"query","class":"OC\\AppFramework\\DependencyInjection\\DIContainer","type":"->","args":["OCA\\WebAppPassword\\Controller\\ShareAPIController"]},{"file":"/var/www/html/lib/private/Route/Router.php","line":338,"function":"main","class":"OC\\AppFramework\\App","type":"::","args":["OCA\\WebAppPassword\\Controller\\ShareAPIController","preflightedCors",["OC\\AppFramework\\DependencyInjection\\DIContainer"],["webapppassword.shareapi.preflighted_cors"]]},{"file":"/var/www/html/lib/base.php","line":1050,"function":"match","class":"OC\\Route\\Router","type":"->","args":["/apps/webapppassword/api/v1/shares"]},{"file":"/var/www/html/index.php","line":49,"function":"handleRequest","class":"OC","type":"::","args":[]}],"File":"/var/www/html/apps/files_sharing/lib/Controller/ShareAPIController.php","Line":124,"message":"Cannot assign null to property OCA\\Files_Sharing\\Controller\\ShareAPIController::$currentUser of type string","exception":{},"CustomMessage":"Cannot assign null to property OCA\\Files_Sharing\\Controller\\ShareAPIController::$currentUser of type string"}}