Can't use SD card on MT7621

The hardware is a HooToo HT-ND001, also known as the MTC 1201. OpenWrt is yesterday's trunk build. I think I have all storage related kmods installed, including mt76-core, kmod-mmc, scsi-core, scsi-generic, sdhci, sdhci-mt7620. I tried both 2 GB and 8 GB SanDisk cards. The card is detected and the filesystem can be mounted using a SD to USB adapter plugged into the USB port.

Upon plugging the card into the SD slot, this error occurs 4 times and nothing else in the log.

[42716.386281] msdc0 -> XXX sdc_busy timeout: before CMD<55> <- msdc_command_start() : L<860> PID<kworker/0:2><0x1b7>
[42716.407176] mmc0: error -145 whilst initialising SD card

Is this something that is known to not work?