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.
Any additional help would really be appreciated!