Thanks for the log, there seems to be a multi-step process involved on the newer ESP8266 toolchains, however it looks reasonably simple.....

It is often best to inspect the one that is there and clone it to a location outside of the board package (e.g. your project folder where the main INO is), so it is not lost with the next board package version update.

So for example, we can override the Flash Size selection properties from the board menu for a single selection:

1) Ensure the Board Option "Flash Size" to be "Mapping defined by Hardware and Sketch" (as it is the one selected to override in the attached entries).

2) Copy the Current files to your project folder:
a) ools/sdk/ld/eagle.flash.auto.ld
b) ools/sdk/ld/eagle.app.v6.common.ld.h
c) ools/sdk/ld/eagle.app.v6.common.ld.vtables.h

3) Add a Local board.txt to your project, and paste in the contents of the attached text file.

That should be it I believe....

Hello again Simon,

now after using the data with user defuined variable, I encountered some unexpected behaviour. 
The 
rtcData data1 = {
     "Berni167",
     0,
     "/rec_2024-12-31__23-59-59.csv",
     0,
     0xFFFF
};

is not inititialized when adding __attribute__((section(".rtcDataSection")))

although I have the expected address defined in the eagle.app.v6.common.ld.h , but no data written on this memory, or atleast not the expected one.

My workaround was just define the data1 in the setup() and this worked. I investigated some more and there should be some startup (startup.s) file which tells the processor what to do before main() and where to copy data from e.g. flash to RAM. Do I miss out some file where I have to define the init values from rtcData data1 , or is there some better way to clarify the init values for data1?

Many thanks  in advance,
Christoph