Hi!

Answered in the Pro Ticket, but for other users, issue seems related to a broken Joomla upgrade (i don't have yet the reason for this) but for some users, Joomla 3.5 upgrade fails to replace some files by new ones, and with a partial file update.
The reason a bug appears in iCagenda is not related to iCagenda, but to a missing new file for file upload field, which one is used in iCagenda, and using Joomla library.
So you may have this issue with other extension using the form field file upload.

In the same time, other files may nto be correctly updated... (so possible other issues...)

A good way to fix a broken Joomla install, is to install it again.

Follow the method C in this documentation:
docs.joomla.org/J3.x:Upgrading_from_Joomla_3.4.x_to_3.5/de

And use this zip file for re-install: "Update from Joomla! 2.5, 3.0, 3.1, 3.2, 3.3 or 3.4"
here: github.com/joomla/joomla-cms/releases

DON'T FORGET to do all steps, step by step!

Hope this will help some users with broken Joomla 3.5.1 update.

Cyril