This on WinXP SP2 + WMF11.
I have a recollection of no such problem on a previous version.
I hope a quick fix is possible esle I'll have to revert.
EDIT: Same failure if I simply reselect the current Audio codec: value of WMA - the operation changes Type of bitrate and then OK gives:

Workaround from Mizery_Made: switch to skinned!!
EDIT: Persists in 1261.