Describe the bug
The xtb 6.6.0 release seems to introduce some problematic behaviour with the --acc flag. A numerical hessian calculation may arbitrarily hang, segfault or crash with erros that numerical thresholds can not be reached.
We found this behaviour when updating xtb in NixOS-QChem, but I can reproduce the behaviour also on Debian.
Interestingly everything works fine when the --acc flag is removed. The actual value of acc does not matter for the behaviour at all.

To Reproduce
Steps to reproduce the behaviour:
After building on a fresh Debian OCI-Container via

sudo apt install gfortran cmake build-essential git libopenblas-dev
git clone https://github.com/grimme-lab/xtb.git && cd xtb
cmake -B build -DCMAKE_BUILD_TYPE=Release
make -C build
make -C build test
sudo make -C build install

or with Nix via nix run github:sheepforce/nixos-qchem/xtb#xtb -- xtb.xyz --hess --input xcontrol --chrg 0 --uhf 0 --gfn 2 --acc 1.0

1. happens with input (include input files)
   xtb.xyz

4

  H -0.35738652 -2.42732105  0.01869782
  H -0.55575507  0.31681832  1.27512527
  O -0.71404094 -1.41851105  0.01869782
  O -0.20069873 -0.69255477  1.27610279

xcontrol

$write
            json=true
        $end

3. start xtb with (all the options here)

xtb xtb.xyz --hess --input xcontrol --chrg 0 --uhf 0 --gfn 2 --acc 1.0

4. run xtb with your options and the --verbose flag
5. output showing the error
   vebose.txt

The behaviour is not reproducible. The very same calculation may error with the shown messages, segfault or hang infinitely from the same input with any number of OpenMP threads.

Expected behaviour
Reproducible behaviour, terminating calculation and correct behaviour with respect to the --acc flag.

Additional context
The problem seems not to occur with the downloadable binary, which I assume is built with the Ifort compiler.
The BLAS implementation does not matter (tested OpenBlas, MKL and a generic BLAS).
XTB and all its dependencies pass all cmake tests.