- Master 4.19.93 on mvebu with DSA
- ip-bridge 5.4.0-1
- swconfig not installed
- kmod-swconfig 4.19.93-1 installed as dependency to kmod-switch-mvsw61xx
Enabling STP on the bridge I am observing the same as previously reported
Since a search in the public domain does not unearth any hints
it would appear somewhat particular to OpenWrt or else it would likely have popped up somewhere else already.
errno 34 produces
ERANGE 34 Result not representable
and does not shed much light on the matter either.
Perusing the OpenWrt source code for
to set attribute turns up 
nl_perror(-retval, "Failed to set attribute");
but that files pertains to the swconfig package which is not installed though.
Bit of an enigma what this error represents and how to remedy it.