UPDATE FROM MABULA
I have had a very rough 2 months unfortunately health wise. I was struck 3x in a row with bacterial infections. The second infection occurred  after a routine hospital checkup and I became very sick. I had to rest and take a lot of antibiotics. Once I was recovering and restarting work 1 month ago, I again became sick. The infection was not yet gone, so even more antibiotics and rest was needed. Needless to say, it took a lot of my energy and I needed a lot of rest.
Finally, the infection is really gone and my energy is coming back step-by-step now and I have started work again. I am terribly sorry to have kept you waiting for support. I will address all outstanding questions and e-mails step-by step and will be on the forum daily from today.
APP 2.0.0-beta47 will come soon as well, a lot of work was already completed before I became very ill, so the release is also nearly ready for you. Beta47 will be much faster actually. Many workflows will be more than 2x faster, mosaics can even be 10x faster than before because registration really received a major boost... all compared to beta46. Â Before I release it, I will make sure that everything is working properly and then I will release it.
@tschuh Do you get the error as well when saving to FITS? Is your working folder on the internal hard drive or an external one? If external, what type is it? Normal HD or SSD?
@tschuh So you probably get this error after processing a FITS file in APP, correct? Can you explain exactly what steps you do to get to this error and upload the FITS file that was used to
https://upload.astropixelprocessor.com/
using upload1 as both username and password? Please create a folder called tschuh_tiff_error and put the FITS file in there. Then we will have a look.
@wvreeven I was about to upload my FITS when I figured out what the problem was. My A/V software (Avast) was *silently* blocking not the fwrite() but the APP application itself. A recent update made that block more vocal and it was highlighted as a blocked application, false positive for ransomware of all things. I've whitelisted the app and all appears to be working correctly again.

