[TYPO3-dev] Error "Extension [...] is not available"

[Michel Mix]

I have two fresh projects (development environment, TYPO3 6.2.9). In project 1 I make the most basic extension 'user_test' with just an ext_emconf.php. The extension can be installed without problems. 

Next I download the extension as zip, and try uploading it in the EM of the second project. Now it flashes an error message: "Extension user_test_0.1_201502151708 is not available". I get the same error in existing older projects.

Any ideas why? Did I make a mistake, or is this a bug?

The ext-folder has 777 rights. I can install other extensions in the 2th project, both t3x and zip-extensions. Copying the 'user_test' manually and then installing it works too. The location where the exception is thrown is sysext/extensionmanager/Classes/Controller/ListController.php:85, but that didn't help me yet to figure out what went wrong.
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Medellín, Colombia | Certified TYPO3 Integrator | Bronze member of the TYPO3 Association