OpenWRT xRX200 DSL stops forwarding data

Running a very recent version of OpenWRT (SNAPSHOT r19597-65258f5d60 / LuCI Master git-22.123.46500-111c551) I observe regular events where the DSL link remains up but data transfer stops completely as PADI messages received are not being forwarded.
Running OpenWRT on a ZyXEL P-2812HNU-F1 whch runs in bridge mode. PPPoE session terminated in a seperate PFSense firewall.

What I observed today (lost connection to internet again) is that the ptm_mailbox_isr interrupt count did not increase anymore. Unloading the ltq_ptm_vr9 module (best I could think of other than rebooting) resulted in my connectivity to the Zyxel temporarily lost. Setting up a new SSH session (which worked without rebooting) left me only with the option to reboot eventually. After reboot PPPoE session was re-established.

Any idea what might be happening here?