I want to share an interesting case about updating the firmware over FOTA.. I have several nodes on a RS485 bus with memory chips. Recently I decided to update the firmware on several at once and ran into an unexpected problem - repeated attempts to update constantly ended up with a checksum error !OTA:FWP:CRC FAIL. At the same time, I usually test all new nodes for FOTA firmware updates before installing. I tried to understand what the problem might be and came to an unexpected result - in my case, disabling the directive OTA_EXTRA_FLASH_DEBUG, which I had been using for a long time, when I first tested updates, helped. After disabling this directive, all firmwares were updated the first time. Very strange, but nevertheless ...

@guillermo-schimmel Thanks a lot ! I got it all up and running node a couple of nodes. But now I confused myself . I got an Arduino Nano as a node that also acts as a repeater, what bootloader can I push into that one? I assume anycahnnel and 16MHz should do the trick since it got that crystal attached. And also one thing that is not really clear in my head, I set the "Fuses&Lock bits" with the AVRDudess for my 328P barebone. But do I really need that? . If I load the bootloader in ARduini IDE for example I specify all those settings in the "board.txt" file. Or is that just to tell the IDE what settings is on the chip ? If I clear these two things up I think I got everything sorted now =)