Unknown function at 0x00000A80

0X00000A80 is the hardfault in the softdevice (MBR). 
There are a lot of threads here discussing this. You should start your application in debugger mode and start to trigger this hardfault so that you can debug the hardfault 

How do I know what the problem is? The IDE doesn't give me much information.

If you enable logs, the hardfault handler should normally give the address of the instruction that caused the hardfault. If not, there could be stack corruptions making the exception stack useless to debug. In case of memory corruptions, there is no easy way to debug apart from trying to narrow down the problem in the debugger sessions and adding lot of logs.

Also check if there are any possible stack overflows if your application RAM usage is getting close to the limit of available memory.

Also check if there are any possible stack overflows if your application RAM usage is getting close to the limit of available memory.

Maybe I figured out where the problem was.
Perhaps the WDT timer expires and restarts the microcontroller. It entered the bootloader and gave me the error message.
I will try to increase the time of the WDT. For now it works, I'll keep you updated.
Thank you very much.

Thanks for updating this thread on the details of the possible problem. 

Stefano1984 said:

I'll keep you updated.

Great, Thank you.