This problem have now been solved thanks to Esris support team.
Il explain it if someone else has the same problem:
With Fiddler I could see that server responded "498 Invalid Token" when request were sent for new tiles after beeing in sleppmode for more then an hour.
This was becouse the token were generated in Arcgis Server with a lifespan of one hour.
This was fixed by replacing
IdentityManager.Current.GenerateCredentialAsync(ONLINE_BASEMAP_TOKEN_URL, USERNAME, PASSWORD);
cred.ExpirationDate = DateTime.Now.AddDays(14);
IdentityManager.Current.TokenValidity = 14 * 24 * 60;
with
GenerateTokenOptions options = new GenerateTokenOptions();
options.TokenValidity = 60 * 24 * 14;
options.TokenAuthenticationType = TokenAuthenticationType.ArcGISToken;
var cred = await IdentityManager.Current.GenerateCredentialAsync(ONLINE_BASEMAP_TOKEN_URL, USERNAME, PASSWORD, options);
Another solution could have been to change the lifespan of the token on the server