[imp] ERR: HORDE IMP is marked as authenticated, but no credentials can be found in the session.

[Alex]

Dear all,

using Horde Webmail on Ubuntu 16.04 with mariadb and php 5.6. Since
updating with apt-get upgrade I cannot use IMP anymore because I get the
following error:

IMP is marked as authenticated, but no credentials can be found in the
session.

The error log shows the following message:

ERR: HORDE [imp] IMP is marked as authenticated, but no credentials can be
found in the session. [pid 489 on line 628 of "/usr/local/share/horde/imp/
lib/Imap.php"]

IMP is version 6.2.17 and horde is 5.2.13

Any idea where I can start looking to solve the error message? Any help
would be appreciated.

Regards,
alex