I think the key name is not very helpful, because as far as I understand, it was generated together with a timestamp, so every time I start another upload, the key changes.
But I can give better information’s now. I have read the debug manual and had generated a Logfile.
This is the part, where the UploadStash failure happened:
UploadStash::stashFile stashing file at '...\Temp\php2F21.tmp'
[error] [add76eab3e626d15dc4f62d8] /mediawiki/api.php ErrorException from line 249 of ...\mediawiki\includes\upload\UploadStash.php: PHP Warning: preg_match(): Compilation failed: invalid range in character class at offset 4
#0 [internal function]: MWExceptionHandler::handleError(integer, string, string, integer, array)
#1 ...\mediawiki\includes\upload\UploadStash.php(249): preg_match(string, string)
#2 ...\mediawiki\includes\upload\UploadBase.php(1124): UploadStash->stashFile(string, string)
#3 ...\mediawiki\includes\upload\UploadFromChunks.php(124): UploadBase->doStashFile(User)
#4 ...\mediawiki\includes\upload\UploadBase.php(1070): UploadFromChunks->doStashFile(User)
#5 ...\mediawiki\includes\upload\UploadFromChunks.php(76): UploadBase->tryStashFile(User, boolean)
#6 ...\mediawiki\includes\api\ApiUpload.php(315): UploadFromChunks->tryStashFile(User, boolean)
#7 ...\mediawiki\includes\api\ApiUpload.php(212): ApiUpload->performStash(string)
#8 ...\mediawiki\includes\api\ApiUpload.php(132): ApiUpload->getChunkResult(array)
#9 ...\mediawiki\includes\api\ApiUpload.php(104): ApiUpload->getContextResult()
#10 ...\mediawiki\includes\api\ApiMain.php(1579): ApiUpload->execute()
#11 ...\mediawiki\includes\api\ApiMain.php(535): ApiMain->executeAction()
#12 ...\mediawiki\includes\api\ApiMain.php(506): ApiMain->executeActionWithErrorHandling()
#13 ...\mediawiki\api.php(94): ApiMain->execute()
#14 {main}