Error message after upgrade to 18.2 or Transmission install

[riotshield]

I upgraded the cloudbox to 18.2 and at the same time almost installed the Transmission addon.

I am not sure what caused it but after the above actions I get the following error message at the top of the page above everything
"Warning: session_start(): open(/tmp/sess_f26dfaa076199bacf0562278026246db, O_RDWR) failed: Read-only file system (30) in /usr/htdocs/home.hm on line 2 Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /usr/htdocs/home.hm:2) in /usr/htdocs/home.hm on line 2 "

When I woke up today, the cloudbox was disconnected from the network, maybe a random restart during the night?

What exactly the error means, and is there a way to fix it? I have downgraded to 18.1 and removed transmission but it did not help.