Mh, do you use Arduino IDE 2?
Yes, it's a weakness of Arduino IDE, that you can't specify the exact libraries.
But I think, server.h is only one part of my outsourced code parts in src.
All my used Libraries are the newest from the Library Manager beside "ESPAsyncWebServer", use my version. Because the version in the manager is not longer compatible to the esp core code.
Gruß Zeppelin

I found the reason for the problem: Your filename server.h is very unfortunate as the same filename is being used by the arduino framework code within NetworkServer.h:
#include "Server.h"
When compiling the project the name collision causes the compiler to use your server.h file instead of the one contained in the arduino framework:

In file included from /Users/.../Library/Arduino15/packages/esp32/hardware/esp32/3.0.7/libraries/Network/src/NetworkServer.h:22, from /Users/.../Library/Arduino15/packages/esp32/hardware/esp32/3.0.7/libraries/Network/src/Network.h:13, from /Users/.../Library/Arduino15/packages/esp32/hardware/esp32/3.0.7/libraries/WiFi/src/WiFiGeneric.h:39, from /Users/.../Library/Arduino15/packages/esp32/hardware/esp32/3.0.7/libraries/WiFi/src/WiFiSTA.h:29, from /Users/.../Library/Arduino15/packages/esp32/hardware/esp32/3.0.7/libraries/WiFi/src/WiFi.h:33, from /Users/.../Documents/Arduino/libraries/ESPAsyncWebServer/src/ESPAsyncWebServer.h:32, from /Users/.../proj/MBusino/src/MBusino/MBusino.ino:20: /Users/.../proj/MBusino/src/MBusino/Server.h: In function 'void setupServer()': /Users/.../proj/MBusino/src/MBusino/Server.h:2:3: error: 'server' was not declared in this scope; did you mean 'Server'? 2 | server.on("/", HTTP_GET, [](AsyncWebServerRequest *request){

My solution to fix this issue is renaming your server.h file to httpserver.h - maybe you should consider doing the same.