Common causes are a server that is down for maintenance or that is overloaded.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/503
#0 index.php(68): RssBridge->main()
#1 lib/RssBridge.php(39): RssBridge->{closure}()
#2 lib/RssBridge.php(37): CacheMiddleware->__invoke()
#3 middlewares/CacheMiddleware.php(44): RssBridge->{closure}()
#4 lib/RssBridge.php(37): ExceptionMiddleware->__invoke()
#5 middlewares/ExceptionMiddleware.php(17): RssBridge->{closure}()
#6 lib/RssBridge.php(37): SecurityMiddleware->__invoke()
#7 middlewares/SecurityMiddleware.php(19): RssBridge->{closure}()
#8 lib/RssBridge.php(37): MaintenanceMiddleware->__invoke()
#9 middlewares/MaintenanceMiddleware.php(10): RssBridge->{closure}()
#10 lib/RssBridge.php(37): BasicAuthMiddleware->__invoke()
#11 middlewares/BasicAuthMiddleware.php(13): RssBridge->{closure}()
#12 lib/RssBridge.php(37): TokenAuthenticationMiddleware->__invoke()
#13 middlewares/TokenAuthenticationMiddleware.php(10): RssBridge->{closure}()
#14 lib/RssBridge.php(34): DisplayAction->__invoke()
#15 actions/DisplayAction.php(54): DisplayAction->createResponse()
#16 actions/DisplayAction.php(89): RoosterTeethBridge->collectData()
#17 bridges/RoosterTeethBridge.php(91): getSimpleHTMLDOM()
#18 lib/contents.php(162): getContents()
#19 lib/contents.php(117): HttpException::fromResponse()
#20 lib/http.php(36)