What does the error tell us?

Problemereignisname:  BEX64
Fehlermodulname:  StackHash_756f
Ausnahmeoffset:   PCH_A3_FROM_GTAVLauncher+0x00000000000A5B29
Ausnahmecode: c0000005
Ausnahmedaten:    0000000000000008

0xc0000005 is an exception of type (memory) Access Violation, meaning that the game tried to access memory that did not belong to it ^[1]. This might be a software error (bug) in the game itself, or in one of the libraries it uses ^[2].

This particular error is probably raised by DEP (Data Execution Prevention):

A user-mode DEP exception results in a STATUS_ACCESS_VIOLATION (0xc0000005) on Windows systems. The first parameter of ExceptionInformation that is located inside the EXCEPTION_RECORD structure contains the type of access violation that occurred. A value of 8 for ExceptionInformation[0] indicates the access violation was an execution violation.

Note that this (8) is the value of your Exception Information (Ausnahmedaten).

How do we fix this?

Because this is a software error, the usual course of action is to wait until the publisher updates the software. This is the safest course to take, but may take a few days or weeks.

Another option is to exclude this application from DEP. However, because the DEP error is a symptom of the problem, and not itself the problem, there is no guarantee that adding an exception will resolve your problem.

From the Wikipedia:

These problems may be prevented by disabling DEP, but this increases vulnerability of the system to malware. DEP can be turned off on a per-application basis, or turned off entirely for all non-essential Windows programs and services. Microsoft recommends that DEP not be globally disabled where an application malfunctions due to incompatibility with DEP. Instead, the supplier of the offending software should be contacted for an updated version that does not violate DEP; until the problem is corrected DEP may be disabled on an exception basis for the offending application only.

^{[1] ... or that was not marked to be so accessed, e.g. trying to execute code from memory not marked as executable.}

^{[2] However, Windows was unable to determine where exactly it went wrong, which is why it took a "Stack Hash".}

After trying EVERYTHING, I wanted to reinstall it. I didn't had an idea why this should work (as the exe file has been redownload often), but it was from deepest desperation.

The GTA V Installation Tool (basically the setup) offered me a repair function.

Repair functions often caused problems a long time ago in gaming, as they have overwritten already patched files with the old ones from the physical disc.

I gave it a shot. It repaired about 5 minutes, then the launcher started. 50 MB have been downloaded and after that the most recent patch (~350 MB).

It is now working.

The GTA V Installation Tool (if a non-Steam version) can be obtain via the Rockstar Social Club website and an activated GTA V key on your account.

This is a common issue as of recently. It is regarding some conflicts with either programs or hardware. The community has been on Rockstar's case about this with no news as of yet. Example.

But there are plenty of YouTube videos regarding possible fixes for it such as this one. Although you mentioned you tried disabling your graphics card already. Players have even wiped their hard drives to no avail.

Other than that there is no "offical" way of fixing it but hopefully Rockstar fixes it soon.