The update is from Joomla 3.4. I restored a backup of the site with joomla 3.4: no problems. Updating this backup to 3.81 (as is the live website): the problem returns. An separate backup with joomla version 3.8 also has the problem.
The problems occur in frontend and back-end, although they are slightly different as I stated in the original post. Clearing the browser-cache doesn't solve the problem. I have also tried different browsers (firefox, chrome, iPad) with the same results.
The info you asked for:
PHP Version : 7.0.22
MySQL Version : 5.5.5-10.1.26-MariaDB
Server Information : Linux ikdirectadmin12.ikoula.com 3.16.0-4-amd64 #1 SMP Debian 3.16.36-1+deb8u2 (2016-10-19) x86_64
Fix jQuery? : Yes
Load JEvents Bootstrap CSS? : Yes
Max Input Vars ? : 1000
Club code set? : No
User Agent : Mozilla/5.0 (Windows NT 6.1; WOW64; rv:56.0) Gecko/20100101 Firefox/56.0
Joomla : 3.8.1
component_com_jevents : 3.4.43
component_com_jce : 2.6.15
module_mod_jevents_cal : 3.4.43
module_mod_jevents_custom : 3.4.43
module_mod_jevents_filter : 3.4.43
module_mod_jevents_latest : 3.4.43
module_mod_jevents_legend : 3.4.43
module_mod_jevents_switchview : 3.4.43
plugin_content_jevents : 3.4.43
plugin_finder_jevents : 3.4.43 (not enabled)
plugin_installer_jeventsinstaller : 3.4.43 (not enabled)
plugin_search_eventsearch : 3.4.43
plugin_system_gwejson : 3.4.43