Still continuing. I rebooted, but it didn't fix the problem.
About 15 / min of those messages in syslog (dmesg, /var/log/syslog)

# dmesg|tail
[29227.566741] dspbridge: timed out waiting for mailbox
[29227.582458] dspbridge: timed out waiting for mailbox
[29228.206298] dspbridge: timed out waiting for mailbox
[29228.229766] dspbridge: timed out waiting for mailbox
[29228.518829] dspbridge: timed out waiting for mailbox
[29228.535491] dspbridge: timed out waiting for mailbox
[29237.112548] dspbridge: timed out waiting for mailbox
[29237.128173] dspbridge: timed out waiting for mailbox
[29237.449554] dspbridge: timed out waiting for mailbox
[29237.465270] dspbridge: timed out waiting for mailbox