Reconnecting secondary node in shared-nothing active-passive DRBD cluster after secondary reboot

Question

I'm using DRBD (config below) and tried to test reliability of the setup.

I have rebooted secondary node (host1) and noticed it went into this state:

host1:
 0:r0/0  WFConnection Secondary/Unknown UpToDate/DUnknown 

host2:
 0:r0/0  StandAlone Primary/Unknown UpToDate/DUnknown lvm-pv: vgr0 1861.65g 40.00g

drbd service was running on primary, it also started on secondary. However, anything I tried on secondary failed to reconnect it:

drbdadm adjust all

drbdadm disconnect r0

drdbadm connect all

All commands ended with:

Failure: (102) Local address (port) already in use.

Finally, I have restarted drbd service (service drbd restart) on a primary. Only that reconnected the service:

host1:
 0:r0/0  Connected Secondary/Primary UpToDate/UpToDate 

host2:
 0:r0/0  Connected Primary/Secondary UpToDate/UpToDate lvm-pv: vgr0 1861.65g 40.00g 

Why is that? Can I recover from WFConnection without restarting the service on primary?

Resource definition:

resource r0 {
        protocol C;
        startup {
                wfc-timeout  15;
                degr-wfc-timeout 60;
        }
        disk {
                on-io-error     detach;
                c-fill-target   10M;
                c-max-rate      700M;
                c-plan-ahead    7;
                c-min-rate      4M;
        }
        net {
                # max-epoch-size  20000;
                max-buffers       36k;
                sndbuf-size       1024k;
                rcvbuf-size       2048k;
                after-sb-0pri    discard-zero-changes;
                after-sb-1pri    discard-secondary;
                after-sb-2pri    disconnect;
                rr-conflict      disconnect;                
        }
        syncer {
                rate                    400M;
                al-extents              6433;
        }
        on host1 {
                device /dev/drbd0;
                disk /dev/sdc;
                address 10.0.0.2:7788;
                meta-disk internal;
        }
        on host2 {
                device /dev/drbd0;
                disk /dev/sdc;
                address 10.0.0.3:7788;
                meta-disk internal;
        }
}

I'm using /dev/drbd0 as LVM volume:

% pvdisplay  /dev/drbd0
  --- Physical volume ---
  PV Name               /dev/drbd0
  VG Name               vgr0
  PV Size               1.82 TiB / not usable 3.79 MiB
  Allocatable           yes 
  PE Size               4.00 MiB
  Total PE              476583
  Free PE               466343
  Allocated PE          10240
  PV UUID               JC0Us5-jxC1-9u2F-Wsyp-toJy-E2J4-sXu8Id