Sync disabled: doesn't work at all

[TroudhuK]

The YouTube subscriptions button does nothing, and the opml import fails.

Error in console:

12:56:38,885 Uncaught (in promise) Error: QuotaExceededError: storage.sync API call exceeded its quota limitations. undefined
    apply self-hosted:2159
    applySafeWithoutClone resource://gre/modules/ExtensionCommon.jsm:614
    wrapPromise resource://gre/modules/ExtensionCommon.jsm:889
    (Asynchrone : promise callback)
    wrapPromise resource://gre/modules/ExtensionCommon.jsm:850
    wrapPromise resource://gre/modules/ExtensionCommon.jsm:849
    callAsyncFunction resource://gre/modules/ExtensionCommon.jsm:1091
    callAsyncFunction resource://gre/modules/ExtensionChild.jsm:701
    callAndLog resource://gre/modules/ExtensionChild.jsm:677
    callAsyncFunction resource://gre/modules/ExtensionChild.jsm:700
    stub resource://gre/modules/Schemas.jsm:2679
    7 moz-extension://8a3dbbca-7159-48e4-a7a1-83ba181c0335/manage/manage.js:1
    AsyncFunctionNext self-hosted:684