MOX SDIO not working on TOS 6 (HBL)

Since upgrade to TOS 6 SDIO is not working anymore. Creating SSID via luci or FORIS doesn't make any difference as well as rebooting (warm or cold). Basic error in syslog is

Apr 11 13:40:45 PatIsa_AP_OG1 kernel: [  937.284750] mwifiex_sdio mmc0:0001:1: CMD_RESP: cmd 0xb1 error, result=0x1
Apr 11 13:40:45 PatIsa_AP_OG1 kernel: [  937.291952] mwifiex_sdio mmc0:0001:1: Failed to start the BSS
Apr 11 13:40:45 PatIsa_AP_OG1 kernel: [  937.298028] mwifiex_sdio mmc0:0001:1: Failed to start AP

Changes since (working) 5.3.3: Replaced WLE1216v5-20 by AW7915-NP1, added MOX D module with 10GBE-module. SDIO is attached (same as before) to external antennas via diplexers.