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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 12.0.2): 16.0.4
Operating system and version (eg, Ubuntu 17.04): OpenSUSE Leap 15.1
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.33
PHP version (eg, 7.1): 7.2.5
The issue you are facing:
After update to Calendar 1.7.1, the below error appeared on my integrity check.
I’ve checked, it looks for signature.json file in appdata but it’s not there, so it drops this error. Weird thing is I’ve checked multiple versions of calendar and none of them had this file. Are there others facing this issue? Should I drop an issue on Github?
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
- Signature data not found.
- EXCEPTION
Raw output
Array
(
[calendar] => Array
(
[EXCEPTION] => Array
(
[class] => OC\IntegrityCheck\Exceptions\InvalidSignatureException
[message] => Signature data not found.
)
)
)
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
- Upgrade calendar to 1.7.1
The output of your Nextcloud log in Admin > Logging:
{"reqId":"sUVgwUNBoPWSLEFBP3mO","level":3,"time":"2019-09-09T05:04:00+00:00","remoteAddr":"","user":"--","app":"PHP","method":"","url":"\/occ","message":"file_get_contents(\/srv\/www\/nextcloud\/apps\/calendar\/appinfo\/signature.json): failed to open stream: No such file or directory at \/srv\/www\/nextcloud\/lib\/private\/IntegrityCheck\/Helpers\/FileAccessHelper.php#41","userAgent":"--","version":"16.0.4.1"}
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
PASTE HERE
The output of your Apache/nginx/system log in /var/log/____:
PASTE HERE