Probably some file system corruption on your SD card.
What does it mean? Why are there no files only there and with the above command everything is fine?
I didnât want to mention it, but the machine I mentioned at the very beginning of the post has also not worked for two days. It lasted a total of 60 days without a problem, but when I go to the place I will see what happened and share.
Probably you have a faulty / cheap SD card which degrades fast. Critical data can not be read anymore which can lead to every sort of problems, like an empty boot folder. Also certain power loss can lead to such data corruption.
Depending on which software / apps you use there is maybe also a lot of writing to the SD card, which degenerates the flash memory also. If you use a card of low quality card you can run pretty fast into such problems.
The fsck command just checks the card for such corrupt data and tries to restore it.
An alternative would be to use a higher quality card, USB flash drives or (the best solution) SSDs or HDDs via USB.
Dietpi also tries to prevent such corruption e.g. by using RAMlog, which writes your logs to the RAM instead of SD card.
What card do you recommend? Because something makes me think youâre on the right track.
I have a third machine that today made 100 days since it was installed with Mixza sd card from Aliexpress.
Basically your SD card has 2 separate partition. One of them is holding your boot file system. During startup the system is going to mount the boot file system into /boot. If this not working, you boot folder stays empty. The obove command is going to fix file system errors and this seems to be enough on your system to boot correctly.
If possible I would try to avoid using SD cards. They are not designed to operate 24/7 and it is not that uncommon that they die sooner or later. It depends on load, r/w operation and of course on quality of SD card used. On RPi device you could check if you are able to use USB pen stick as boot media instead of SD cards.
Got it, I thought it booted. As C:\ like a boot in dos
I am very new to linux, please excuse me.
I use brand new cards but they are really cheap. The entries in them should not be more than the theoretical maximum (rather, we only write temperature twice a day), the cards I currently use are ADATA 32GB, the most common.
It should be something like a SanDisk Ultra microSD, A1, Class 10
I understand the message, I have been looking for a micro sd card that is like an ssd for a long time but I have no success.
Maybe you can have a look at RaspiKey eMMC. But thatâs fare away from being cheap 
Today I visited the system I mentioned at the beginning of the post. it turned out everything was working, she just didnât have access to the internet. Unfortunately, I wonât have access to the device until spring. This does not mean that the problem is solved. I have another device that is close and I will keep an eye on it, I will keep you posted. Greetings.
just to avoid a misunderstanding, your issue is more hardware related than software.