I received an identical error to the one above in ankor's post, and applied the patch-files included in the post below. Once applied, I reopened MusicBee and repeated the same actions that had lead to the original "Unexpected Error" and received a new "Unexpected error" as below:
MusicBee v3.1.6434.35538 (Win6.1), 13 Aug 2017 19:42:
System.ArgumentNullException: Value cannot be null.
Parameter name: key
at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
at #=qCpu_xzIw_sj0QCb$1EjSfgRjfHmw_ZQe9elYr8r1aGNxxNjUWo2bUBky$cdijxbi.#=qnyl78zT6ItUlVvs7U0SBIA==(String #=q2b0wxv2qC6Z_B7XSMmpoBw==)
at #=qCpu_xzIw_sj0QCb$1EjSfgRjfHmw_ZQe9elYr8r1aGNxxNjUWo2bUBky$cdijxbi.#=qncjSZ6nmH41xbW6Yp5pJgxkATm9xlxHXYHgpggDrqaY=(String #=qTdq_NjfGjIKv0w6V9sGWAw==)
at #=qCpu_xzIw_sj0QCb$1EjSfgRjfHmw_ZQe9elYr8r1aGNxxNjUWo2bUBky$cdijxbi.#=qf6F9MxOOv3xJTEb7H3k_nP$WmBa0_HlApqK8O8vcrQQ=(String #=qqXHrbWlHena5rl4H$aQo_Q==)
at #=qCpu_xzIw_sj0QCb$1EjSfgRjfHmw_ZQe9elYr8r1aGNxxNjUWo2bUBky$cdijxbi.#=q61i6dqv1BiAH4CgOFREn0UwcjIJxqaaA4uz4R0K9rc8=(#=qsNEQwxE_pjlQVsMLLdoPZA== #=qOY$17z6tw_KC9HSU9u3OQg==)
at MusicBee.Media.StorageManager.#=qWIVY0zQy2ZFJoA7nDaEyI5yoyEShfsNajirwT1pJils=(#=qsNEQwxE_pjlQVsMLLdoPZA== #=qrZNlyoMmRQBm1q50OUomfQ==)
at #=q875noht1xY0yHRu91BuvGV0ucnpILPEwNok$TXbPTnc=.#=qkaL0DlxCjimm_HnjEJysI5N5viM6yoBE9D4Tfdc4bp8=(#=qMmn13t973o1i0psMVD_C3A== #=qGpJhaWXGeokXkh13tPGxdQ==)
at #=q875noht1xY0yHRu91BuvGV0ucnpILPEwNok$TXbPTnc=.#=qFl_6CefZuxlAa3X1F8k3Rw==(SortOrder #=qgPHEu9CZiGdthzr3Sh2TmQ==)
at MusicBee.MusicBeeMain.#=qzxBNO0guikAvUyoPXht08NhkTuSrIV3ovwvHPZHImL0=(Storage #=qJYErO_cDfz68X8CG1D3dLw==, #=qQF$oQH1zfEa1z$gfx1aY3MsXkeaq8_NTm0pRD79rplS0iqih9Eu9cpBTcYsl9bHx #=qN_drByaP2ud2IA4Jk38gsQ==)
at #=qBOg7dI4ItLteHOLh_Br8ykFO_QUsPqFNfN5nCYPMxxk=.#=qYm0gVT1Y6NIkpxxcAFv5dsmhd5qSi$7xpvXJ6XQOFVU=(#=qriD2rWgi8swRDQB7vOaD8A== #=quXHayYrifoaR6juLlk9TpQ==, Boolean #=qArl_JudpOIPHR_tOYMtCCif8HwDIdaeKs2zMzPhVdxQ=)
at #=qiZU$JnsfJpfaegA5nyHHOxBn2siOtzxAnow0_YCnoR4=.#=qriD2rWgi8swRDQB7vOaD8A==.#=qycHYFD0X3Og20o8YbVay1cnMSH2EkD6wrdjXnYplUh0=()