DTLS error: 'too many warn alerts'

res_rtp_asterisk.c:3069 __rtp_recvfrom: DTLS failure occurred on RTP instance '0x7ffb1410a8c0' due to reason 'too many warn alerts', terminating
res_rtp_asterisk.c:7322 ast_rtp_read: RTP Read error: Unspecified.  Hanging up.

I’m looking at establishing a secure call. The messaging is secure and connects over TLS, but with the DTLS setting of this mobile app enabled, this happens after a few seconds (about 6). Without DTLS its fine, but then the media is not encrypted. On the same server, WebRTC Calls are all successful. (I believe WebRTC uses DTLS also)