failed to expand ACL string

My SMTP-Proxy (Sophos UTM 9.004-34) logs sometimes the following log:

2013-01-10 11:37:15 H=(mx.proventhotels.com) [87.193.188.120]:35564 temporarily rejected MAIL <al...@proventhotels.com>: failed to expand ACL string "${lookup dnsdb {>\n; txt= $sender_address_domain}}": lookup of "ptr=87.193.188.120">

I have check the RNDS-Entry; everything is ok; DNS and ptr of the host;
but my astaro this message! I don't unterstand why?

I can diable "do strict RDNS checks" but is this the solution?

Can i clear a spezial database with the ptr entries?
Is this a bug?

Have anyone a idea?

Thanks