I’ve moved it to another server and did a clean install of the latest version, so it’s not an issue anymore.
For the reference, I believe it had to do with the server setup, file ownership in particular. “Nobody” causes trouble with installation like Piwik or WordPress.
thanks for your response matt! unfortunately this did not work. I re-downloaded the current package, stopped apache and reuploaded all files without any error message - the same error still occurs. I also deleted all contents of /tmp - still same result. Clearing browser cache or using alternative browsers didnt help either.
What is strange is, that the same update process worked for every version before v2.15.0 - as other users are experiencing the same issue, I assume that something else than not correctly uploaded files is responsible for this.
Disabling ZenMode plugin worked for me as well, but now the navigation appears in the middle of the screen in IE. Seems to be OK in Chrome. Any ideas ?