add-on update does not work

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Hi!

Concrete5 notified me that an update is available to Advanced Forms add-on which I purchased on use on my website.
I tried to update from version 1.11.5 to 1.13.1.
I keep getting an error message: "Unable to locate this add-on on concrete5.org".
(See attached screenshot.)
I've tried several times, from different browsers, it is always the same.
I contacted the add-on developer as well on this issue.

Thank you in advance for your help.

Peter

Environment info:

# concrete5 Version
5.6.0.2

# concrete5 Packages
Advanced Forms (1.11.5), Breadcrumbs (2.0), Magic Tabs (1.1.0.1), Organica Responsive (1.1), Whale Galleria (1.2.5), Whale Social Links (1.1.3), Zoom Image (1.0.5).

# concrete5 Overrides
None

# Server Software
Apache/2.2.22 (Debian)

# Server API
cgi-fcgi

# PHP Version
5.4.4-14+deb7u11

# PHP Extensions
apc, bcmath, bz2, calendar, cgi-fcgi, Core, ctype, curl, date, dba, dom, ereg, exif, fileinfo, filter, ftp, gd, gettext, hash, iconv, imap, intl, ionCube Loader, json, ldap, libxml, mbstring, mcrypt, mhash, mysql, mysqli, openssl, pcntl, pcre, PDO, pdo_mysql, Phar, posix, Reflection, session, shmop, SimpleXML, snmp, soap, sockets, SPL, standard, sysvmsg, sysvsem, sysvshm, tokenizer, wddx, xml, xmlreader, xmlrpc, xmlwriter, zip, zlib.

# PHP Settings
max_execution_time - 600
apc.max_file_size - 1M
log_errors_max_len - 1024
max_file_uploads - 20
max_input_nesting_level - 64
max_input_time - 300
max_input_vars - 5000
memory_limit - 256M
post_max_size - 32M
sql.safe_mode - Off
upload_max_filesize - 32M
ldap.max_links - Unlimited
mysql.max_links - Unlimited
mysql.max_persistent - Unlimited
mysqli.max_links - Unlimited
mysqli.max_persistent - Unlimited
pcre.backtrack_limit - 1000000
pcre.recursion_limit - 100000
session.cache_limiter - nocache
session.gc_maxlifetime - 7200
soap.wsdl_cache_limit - 5

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