WHMCS released today a security update (TSR-2014-0003), which initially caused some concerns regarding to compatibility of SoluteDNS with this release.
Basic compatibility tests have been performed for WHMCS v5.3.9. There are at this time no indications of any critical conflicts with SoluteDNS v0.1.5 at WHMCS v5.3.9 stable.
Therefor we do give a positive upgrade recommendation.
WHMCS: v5.3.9 (Stable)
Advise: No major concerns, upgrade advised.