The program shows these issues:

Emails are listed in delay, menus are reacting after literally a minute or
more. 
Restarting the program makes no difference, the whole user session must be
terminated, as also restarting Akonadi does not help.
akonadictl fsck does show no error. 

Top does not show kontact as cpu consuming it is just there and apparently
idles and does not respond on requests. The program was normally previously. 

I also found that KDE wallet integration seems to be compromised for IMAP
accounts, I had the case that one of my IMAP accounts lost suddenly the
password. 
When closing the user session I saw the kwallet manager popping up as if it
would have been blocked during access. I will open a separate bug for this. 

The only output from CLI that I do get is as follows:
ntropy@silversurfer:~> kontact
Icon theme "gnome" not found.
Icon theme "ubuntu-mono-dark" not found.
Icon theme "Mint-X" not found.
Icon theme "elementary" not found.
Icon theme "gnome" not found.
No text-to-speech plug-ins were found.
Cannot initialize model with data QJsonObject() . missing: QJsonValue(string,
"urls")
"http://www.kde.org/dotkdeorg.rdf"
"https://www.linux.com/rss/feeds.php"
"http://planetkde.org/rss20.xml"
"http://pim.planetkde.org/rss20.xml"
"https://store.kde.org/content.rdf"
"http://planetkde.org/pt-br/rss20.xml"
"http://planet.kde-espana.org/atom.xml"
"https://fr.planetkde.org/rss20.xml"
kf.xmlgui: Index  24  is not within range (0 -  22 )
kf.xmlgui: Index  26  is not within range (0 -  22 )
kf.xmlgui: Index  24  is not within range (0 -  20 )
kf.xmlgui: Index  26  is not within range (0 -  22 )
kf.xmlgui: Index  26  is not within range (0 -  22 )
"unexpected character" on line 1
kf.xmlgui: Index  26  is not within range (0 -  22 )


I am not aware of particular settings that could cause the problem. 
The previous version of Kontact did not encounter issues.