File Manager shows error

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Hey guys,

When opening the file manager, it shows an error in the size column: "Wrong format... missing 0". Also, when opening the file manager from an editing context, it never stops loading and nothing shows. When uploading a file, I get an empty page, hitting the back button from there shows me that the file I just uploaded replaced the one file I had before...

I've got a feeling that my setup still has a little mistake somewhere. Any hints?

Thanks,
raffomania

Edit: view the attached screenshot to get a sense of the error.

Edit 2: Environment Information:
# concrete5 Version
5.6.3.2
# concrete5 Packages
Slate Theme (1.5.3.1).
# concrete5 Overrides
languages/ja_JP, languages/sk_SK, languages/cs_CZ, languages/sv_SE, languages/it_IT, languages/fa_IR, languages/nb_NO, languages/es_ES, languages/tr_TR, languages/de_DE, languages/fr_FR, languages/es_AR, languages/vi_VN, languages/es_MX, languages/nl_NL, languages/sl_SI, languages/fi_FI, languages/zh_TW, languages/da_DK, languages/ro_RO, languages/es_PE, languages/el_GR, languages/ru_RU, languages/ar, languages/pt_PT
# concrete5 Cache Settings
Block Cache - On
Overrides Cache - On
Full Page Caching - Off
# Server Software
Apache
# Server API
apache
# PHP Version


Edit 3: When creating a new page, I get the following error: "Unknown date format, neither date nor time in 'dd.MM.yyyy HH:mm' found". All my errors are coming from format.php (from the zend library). I'm beginning to suspect there is something wrong with my locale.
Right now I don't know what to do anymore. Any help would be greatly appreciated :)

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