Including SPIFFS-file "from" lib-file fails ("No such file or directory")

I’am having a problem with including the SPIFFS-file. (Also the littlefs-file)

Error message:

include/ImageSystemConnector.hpp:2:10: fatal error: SPIFFS.h: No such file or directory

ImageSystemConnector.hpp contains:

#pragma once
#include "SPIFFS.h"
#include <stdio.h>
#include <cstdint>
#include <iostream>
#include <memory>

I have other files, like my main.cpp-file, which includes SPIFFS.h exactly the same way. If I remove the include in ImageSystemConnector.hpp it compiles (with the including of SPIFFS in the main.cpp) without a problem! I already checked the c_cpp_properties.json, it contains the paths to the source-folder of SPIFFS. (which exists) Of course, it does, otherwise it wouldn’t compile the second way.

I do not really see the difference, why it can not be loaded in my ImageSystemConnector.hpp. Exchanging SPIFFS.h with Arduino.h works. Exchanging SPIFFS.h with LittleFS.h does not work.

include/ImageSystemConnector.hpp is included by a file in the lib-directory. Could this be problematic? I couldn’t really imagine this, because including SPIFFS in the lib-file which includes ImageSystemConnector, works.

Does anyone has an idea, what could cause this problem?

Best regards,