Seems the version 1.4.1 published at apps.owncloud.com has an invalid signature:
Technical information
=====================
The following list covers which files have failed the integrity check. Please read
the previous linked documentation to learn more about the errors and how to fix
them.
Results
=======
- calendar
- EXCEPTION
- OC\IntegrityCheck\Exceptions\InvalidSignatureException
- App Certificate is not valid.
Raw output
==========
Array
(
[calendar] => Array
(
[EXCEPTION] => Array
(
[class] => OC\IntegrityCheck\Exceptions\InvalidSignatureException
[message] => App Certificate is not valid.
)
)
)
I can't reproduce this :/
Let me fire up an oC instance
@RealRancor I tried to fix the cert issue.
Can you validate that calendar2.tar.gz works?
@georgehrke Confirmed, the calendar2.tar.gz from the tag doesn't show this issue. Thanks for the quick fix.
@georgehrke I had the same error and calendar2.tar.gz works for me as well.
Thanks,
Raoul
Thanks for additional confirmation. So lets close this then.
It works yes. But why not in Internet Explorer? It is loading and loading and loading. Look at the screenshot.

is loading and loading.
@DNkRockzzzzZ Unrelated. Please don't hijack issues like this
Is it also possible to fix this issue for the version which an admin can activate under /index.php/settings/apps simply by clicking "activate" in the Frontend (for convenience)?
OC 9.1.6.2
Calendar 1.4.1
EDIT:
... ah now I see, that https://github.com/owncloud/calendar/issues/800 is a newer issue for that. https://github.com/owncloud/calendar/issues/800#issuecomment-305447367 solved my problem. But it's still not very convenient for users upgrading to an OC version with signature failures on calendar version 1.4.1
btw: https://github.com/nextcloud/calendar/issues/196#issuecomment-262800640 didn't work for me. Still had signature revoked error.
Most helpful comment
@DNkRockzzzzZ Unrelated. Please don't hijack issues like this