Streaming replication - 11.5

Hello,
I have two servers connected to the same switch running postgres 11.5

I am trying to replicate one of those servers after a planned work on the
master, so the replica has been lost. It has always worked but now I get
this :

pg_basebackup: could not receive data from WAL stream: server closed the
connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

I don't really understand what the issue is.
I had this issue last week as well in another DC and I had to reboot the
slave to make it working (not sure why it helped)

Do you know what can cause this?

Thank you,
Nicola