Maybe your system files on SD card has been damaged. Which OS (Mac or Windows or Linux) do you use? We will give you the right solution about restoring and get images(fits).
Seems to be 2 issues here, so, as DaveFrance responded "You can open a fits file with Fits Liberator or Fitswork4 both free downloads."
Now, to the other issue from Trampe, the light remains red and cannot connect. ZWO replied that the SD could be corrupt and provided a solution to backing up the image file to another SD card, and that did not fix the issue as per Trampe.
If Trampe did not backup his original image, and if it is corrupt, any backing up now will not fix his problem. (Garbage in, Garbage out) as we say in the IT world.
My question to Trampe is, and I am assuming that the ASIair was working prior, usually a RED light, as I had made this mistake was, using a power supply of 12 volts, but not enough AMPS (2.5 Amps is needed). Is this your case Trampe? AGAIN, I am assuming this ASIair was working perfectly before, but I am just curious.
If so, I can only see a corrupt card and why DaveFrance mentioned it is always better to shutdown the ASIair or any Raspberry Pi unit before removing the SD card. A new image file might be needed to correct the issue and that Trampe try's the solution provided by DaveFrance for opening FIT files once he has ironed out the poissible card issue and gets back his connectivity.
A bit more information of steps taken in a bit more detail would be good to have here from Trampe
Just my opinion.
Hope some of this helps.