[Observium] permissions problem after update

Just finished an update that was a bit overdue (523 --> 540) and now I'm getting various errors while rendering RRD:

"Draw Error" "ERROR: can not parse an empty rpn expression" "RRDTool seems to have generated an empty graph. Please ensure that RRDs are being populated for this device."

While at the same time, some RRDs do render correctly.

On the dashboard the message is different:

"Permission error You have insufficient permissions to view this page"

The poller runs as user:group "observium:obervium" I have run "chown -R observium:observium /opt/observium" out of paranoia, but it didn't help.

What am I missing?

Thanks for your time.