Thanks for your reply.
I believe something's wrong with HTML5 version of IMA because, for instance, AdRules works fine with Flash version but fails in HTML5. I've caught the debug logs from IMA (they simply stop there with no error messages):
[ima.loader.AdsLoaderWrapper] Requesting ads using new ads loader.
[ima.loader.AdsLoaderWrapper] requestAds, processing external request.
[ima.loader.SequentialAdsLoader] Enqueued new request.
[ima.loader.SequentialAdsLoader] Starting request from queue.
[ima.loader.AdsLoaderImpl] creating instance of GptRequestActivity.
[ima.loader.AdSourceFetcher] fetch, starting GPT request activity.
[ima.common.GptCompanionAdService] GPT companion ads service not available.
[ima.request.GptRequestActivity] Started roadblock task.
[ima.request.GptRequestActivity] GPT requests not sent yet.
[ima.request.GptRequestActivity] GPT requests not sent yet.
[ima.request.GptRequestActivity] GPT requests not sent yet.
[ima.request.GptRequestActivity] GPT requests not sent yet.
[ima.request.GptRequestActivity] GPT requests not sent yet.
[ima.request.GptRequestActivity] GPT requests not sent yet.
[ima.request.GptRequestActivity] GPT requests not sent yet.
[ima.request.GptRequestActivity] GPT requests not sent yet.
[ima.request.GptRequestActivity] Pre-request tasks timeout.
[ima.loader.AdSourceFetcher] Creating PlaylistSource.
[ima.loader.AdsLoaderWrapper] dispatchPlaylist_
[ima.adslist.AdBreak] Requesting next ad for time: 0
[ima.loader.AdsLoaderWrapper] requestAds, processing internal request.
[ima.loader.SequentialAdsLoader] Enqueued new request.
[ima.loader.SequentialAdsLoader] Starting request from queue.
[ima.loader.AdsLoaderImpl] creating instance of GptRequestActivity.
[ima.loader.AdSourceFetcher] fetch, starting GPT request activity.
[ima.request.GptRequestActivity] Started roadblock task.
[ima.request.GptRequestActivity] Content {vid: "", cmsid: "3601"} empty.
[ima.loader.AdSourceFetcher] Creating WaterfallAdSource.
[ima.loader.VastAdSource] hasMoreAds, this.ads.length: 1 , this.currentAdIndex: -1 , this.numAdsSent: 0
[ima.loader.VastAdSource] getNextAd, this.currentAdSource: null
[ima.loader.VastAdSource] hasMoreAds, this.ads.length: 1 , this.currentAdIndex: -1 , this.numAdsSent: 0
[ima.loader.VastAdSource] processNextAd_, currentAdIndex: 0, ads.length: 1
[ima.loader.VastAdSource] Setting fallback index to session context: 0
[ima.loader.VastAdSource] processInlineAd, successCallback with 1 ads
[ima.loader.VastAdSource] hasMoreAds, this.ads.length: 1 , this.currentAdIndex: 0 , this.numAdsSent: 1
[ima.loader.AdsLoaderImpl] exhaustAdSource_, calling success for 1 ads
[ima.loader.AdsLoaderWrapper] dispatchAdsLoadedSdkEvent_, ads.length: 1
[ima.adslist.AdBreak] Ad #0 loaded for ad break time: 0
[ima.adslist.AdBreak] Requesting next ad for time: 0
[ima.adslist.AdBreak] Playing next item and waiting for time:0.
[samba.player.html5.js:265 X {b: Object, Nc: function, Pc: function, wb: function, Yc: function…}
[ima.common.GptCompanionAdService] GPT companion ads service not available.
Regards!