AppCenter does not work and constantly crashes

Could you provide logs? You can reach the debug mode of AppCenter by running this in your favorite terminal emulator:

appcenter --debug

Hi, I have the same issue on my desktop built from an iso (elementary OS 0.4 Loki) which I had also used on my laptop where AppCenter works fine. If started from the gui or terminal without admin privileges it's exactly the same issue. If started as an administrator (sudo) it's 'waiting', but still fails:

paul@paul-Aspire-R3610:~$ appcenter --debug
[INFO 14:51:21.101880] Application.vala:155: App Center version: 0.1.4
[INFO 14:51:21.102142] Application.vala:157: Kernel version: 4.4.0-72-generic
Failed to register: Timeout was reached

paul@paul-Aspire-R3610:~$ sudo appcenter --debug
[INFO 15:02:14.416521] Application.vala:155: App Center version: 0.1.4
[INFO 15:02:14.416793] Application.vala:157: Kernel version: 4.4.0-72-generic
[DEBUG 15:02:14.719028] [Gtk] Connecting to session manager
[DEBUG 15:02:14.741752] Added /usr/share/app-info/yaml to metadata search path.
[DEBUG 15:02:14.741961] Added /var/lib/app-info/yaml to metadata search path.
[DEBUG 15:02:14.742228] Caches are up to date.
[DEBUG 15:02:14.742349] Using cached data.
[DEBUG 15:02:15.995076] system/os/package/zathura-pdf-poppler.desktop extends system/os/package/zathura.desktop, but system/os/package/zathura.desktop was not found.
[DEBUG 15:02:15.997524] system/os/package/zathura-pdf-poppler.desktop extends system/os/package/zathura.desktop, but system/os/package/zathura.desktop was not found.
[DEBUG 15:02:16.254223] Client.vala:393: update cache called true
[DEBUG 15:02:16.263765] Client.vala:422: New refresh task
[DEBUG 15:02:16.263992] [PackageKit] adding state 0x1267900
[DEBUG 15:02:16.264367] [PackageKit] role now refresh-cache
[DEBUG 15:02:16.441284] Settings.vala:163: Loading settings from schema 'org.pantheon.appcenter.settings'
[DEBUG 15:02:16.443796] [PackageKit] adding state 0x10773d0
[DEBUG 15:02:16.444148] [PackageKit] role now get-packages
[DEBUG 15:02:17.003450] [PackageKit] notify::connected
[DEBUG 15:02:17.188891] [PackageKit] emit transaction-list-changed
[DEBUG 15:02:17.473759] [PackageKit] emit transaction-list-changed
[DEBUG 15:02:30.161368] [PackageKit] adding state 0x3406900
[DEBUG 15:02:30.161583] [PackageKit] role now resolve
[DEBUG 15:02:30.228737] [PackageKit] emit transaction-list-changed

...(this is where it's waiting after selecting 'Install')...

Installations and updates work fe from the terminal, just not the AppCenter :(

Thank you and regards.
PJ

sorry, I had forgotten and basically I get the same:

sudo appcenter --debug
[INFO 10:25:48.558899] Application.vala:155: App Center version: 0.1.4
[INFO 10:25:48.558941] Application.vala:157: Kernel version: 4.4.0-72-generic
[DEBUG 10:25:48.594798] [Gtk] Connecting to session manager
[DEBUG 10:25:48.596899] Added /usr/share/app-info/yaml to metadata search path.
[DEBUG 10:25:48.596925] Added /var/lib/app-info/yaml to metadata search path.
[DEBUG 10:25:48.596962] Caches are up to date.
[DEBUG 10:25:48.596977] Using cached data.
[DEBUG 10:25:48.599089] Client.vala:393: update cache called true
[DEBUG 10:25:48.600584] Client.vala:422: New refresh task
[DEBUG 10:25:48.600612] [PackageKit] adding state 0x26ee8c0
[DEBUG 10:25:48.600656] [PackageKit] role now refresh-cache
[DEBUG 10:25:48.627248] Settings.vala:163: Loading settings from schema 'org.pantheon.appcenter.settings'
[DEBUG 10:25:48.627534] [PackageKit] adding state 0x27fc3d0
[DEBUG 10:25:48.627559] [PackageKit] role now get-packages
[DEBUG 10:25:48.718462] [PackageKit] notify::connected
[DEBUG 10:25:48.774426] [PackageKit] emit transaction-list-changed
[DEBUG 10:25:48.774616] [PackageKit] emit transaction-list-changed
[DEBUG 10:26:00.230836] [PackageKit] adding state 0x2e12c90
[DEBUG 10:26:00.230918] [PackageKit] role now resolve
[DEBUG 10:26:00.241286] [PackageKit] emit transaction-list-changed
[DEBUG 10:26:04.674210] [PackageKit] adding state 0x3537b30
[DEBUG 10:26:04.674314] [PackageKit] role now search-name
[DEBUG 10:26:04.684471] [PackageKit] emit transaction-list-changed
[DEBUG 10:26:18.131647] [PackageKit] adding state 0x7f6d68001830
[DEBUG 10:26:18.131761] [PackageKit] role now search-name
[DEBUG 10:26:18.142363] [PackageKit] emit transaction-list-changed
[DEBUG 10:26:35.480345] [PackageKit] emit updates-changed
[DEBUG 10:26:43.540205] [PackageKit] cancelling /2562_adbeaccc
[DEBUG 10:26:43.540417] [PackageKit] cancelling /2565_cbbacdcc
[DEBUG 10:26:43.540601] [PackageKit] cancelling /2566_aeaebbcd
[DEBUG 10:26:43.542289] [PackageKit] remove state 0x26ee8c0
[DEBUG 10:26:43.542954] [PackageKit] emit transaction-list-changed
[DEBUG 10:26:43.543125] [PackageKit] cancelled �VP
[DEBUG 10:26:43.544322] [PackageKit] remove state 0x7f6d68001830
[DEBUG 10:26:43.544457] [PackageKit] remove state 0x3537b30
[DEBUG 10:26:43.545106] [PackageKit] emit transaction-list-changed
[DEBUG 10:26:43.545228] [PackageKit] cancelled �+
[DEBUG 10:26:43.545669] [PackageKit] emit transaction-list-changed

The difference today however, is I cannot search or see any other packages, only the ones already installed on my pc.

I also tried a remove and install again, no luck. eventually always times out:

$ ps -ef | grep appcenter
**** 13069 1 3 15:22 ? 00:00:05 appcenter
**** 13156 1440 45 15:25 ? 00:00:01 zenity --question --display :0.0 --class mutter-dialog --title --text <big><b>“appcenter” is not responding.</b></big> You may choose to wait a short while for it to continue or force the application to quit entirely. --ok-label _Wait --cancel-label _Force Quit --icon-name face-sad-symbolic --modal
**** 13171 12521 0 15:25 pts/2 00:00:00 grep --color=auto appcenter

Can you please report this issue on Github: https://github.com/elementary/appcenter/issues

Unmarking this as duplicate since this bug is actually older than the bug it was marked as duplicate of.