The problem was the fact that in my manifest file there weren't some entries: reading the scoreloopUI documentation you can find the "Updating the Android Manifest" section. In this section some entries are provided, but they are not enough! These are the provided entries:
<!-- Defining the ScoreloopCore activities -->
<activity android:name="com.scoreloop.client.android.core.ui.ProxyActivity" android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen" android:configChanges="orientation|keyboard|keyboardHidden|screenSize|smallestScreenSize"/>
<!-- Note: you may need to add more entries here for the payment methods you want to use. -->
<!-- Defining the ScoreloopUI activities -->
<activity android:name="com.scoreloop.client.android.ui.EntryScreenActivity" android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen"/>
<activity android:name="com.scoreloop.client.android.ui.ShowResultOverlayActivity" android:theme="@style/sl_dialog"/>
<activity android:name="com.scoreloop.client.android.ui.component.market.MarketHeaderActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.market.MarketListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.entry.EntryListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.post.PostOverlayActivity" android:theme="@style/sl_dialog"/>
<activity android:name="com.scoreloop.client.android.ui.component.score.ScoreHeaderActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.score.ScoreListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.user.UserAddBuddyListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.user.UserHeaderActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.user.UserDetailListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.user.UserListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.game.GameDetailHeaderActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.game.GameDetailListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.game.GameListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.profile.ProfileSettingsListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.component.profile.ProfileSettingsPictureListActivity"/>
<activity android:name="com.scoreloop.client.android.ui.LeaderboardsScreenActivity" android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen"/>
<activity android:name="com.scoreloop.client.android.ui.framework.ScreenActivity" android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen"/>
<activity android:name="com.scoreloop.client.android.ui.framework.TabsActivity"/>
Searching on the web, I found this scoreloop class (StandardScoreloopManager). At the bottom of this class there's a function that checks the user's manifest file. Some required entries aren't in the documentation above:
"com.scoreloop.client.android.ui.component.achievement.AchievementHeaderActivity"
"com.scoreloop.client.android.ui.component.achievement.AchievementListActivity"
"com.scoreloop.client.android.ui.component.news.NewsHeaderActivity"
"com.scoreloop.client.android.ui.component.news.NewsListActivity"
"com.scoreloop.client.android.ui.component.challenge.ChallengeHeaderActivity"
"com.scoreloop.client.android.ui.component.challenge.ChallengeListActivity"
"com.scoreloop.client.android.ui.component.challenge.ChallengeAcceptListActivity"
"com.scoreloop.client.android.ui.component.challenge.ChallengeCreateListActivity"
"com.scoreloop.client.android.ui.component.challenge.ChallengePaymentActivity"
"com.scoreloop.client.android.ui.BuddiesScreenActivity"
"com.scoreloop.client.android.ui.ChallengesScreenActivity"
"com.scoreloop.client.android.ui.AchievementsScreenActivity"
"com.scoreloop.client.android.ui.SocialMarketScreenActivity"
"com.scoreloop.client.android.ui.ProfileScreenActivity"
Adding these entries in the manifest file solves the issue. Thanks to everyone for the help.