Large PSB file. "file is not compatible with this version of Photoshop."

Silly question time:   Have you turned your computer off and tried again  Not a simple reboot:  turn it off go get a cup of coffee come back and turn it back onand try the file again?

Then prepared the file, which was 84cm x 118,5 at 300dpi

and

They're portraits to be printed in 84 by 118,5 Cm. Each photograph was edited in detail, each one in a different .psd file. Then scaled to the desired size, which made the .psd to be .psb. Each one was saved individually. Each file size was between 5GB and 10GB.

By @defaultn3vk0rzm2je0

A flattened 8 BPC at 84x118.5cm 300 PPI is approx. 397 MB (9,921 x 13,996 px), or around 794 MB at 16 BPC.

At the same print size, at 225 PPI the file size is around 223 MB for 8 BPC flattened, and 447 MB for 16 BPC flattened.

Obviously adding layers increases these base sizes, where you mention sizes of 5-10 GB.

People used to prepare files for Epson inkjets using 360 PPI, or 240 PPI (2/3) of the 360 (720/1440/2880) print resolution for older Epson printers. Many current Epson models use 300 (600/1200/2400) print resolution. Other brands such as Canon or HP generally used 300 (600/1200/2400) print resolution.

Some recommend at a viewing distance of half a metre, that the file would need to be 600 PPI at the high end.

Others recommend 300 PPI at the high end for a viewing distance of half to 1 metre, which is probably more realistic than 600 PPI.

Even 300 PPI could be considered overkill at 1 metre or less viewing distance. 175-200 PPI would often be considered a good range for an input on good quality inkjet photo media. 

Correctly sharpening an image for the content and the output plays a huge role in the perceived quality of detail.

I am not aware of any definitive studies on the topic, with a range of 150-300 PPI often being considered acceptable, there is no problem aiming for the higher 300 PPI value if the original image has sufficient pixels to begin with. If you are upsampling/upscaling/interpolating the resolution from a lower value to hit a magic number of 300, then this probably isn't a good idea.

More on the general topic of resolution and viewing distance:

Then prepared the file, which was 84cm x 118,5 at 300dpi

By @defaultn3vk0rzm2je0

There are three common myths/misconceptions with preparing files for print:

300

and

DPI

and

Printing requires CMYK mode files

Your post mentions 2 of these.

The 300 comes from the upper limit of a traditional halftnone screen reproduction formula, where for a 150 LPI (lines per inch halftone screen), the "quality factor" could be anywhere from x1.5-2 the LPI. This means that the digital image could have an acceptable input value between 225-300 PPI. This is for a traditiona halftone screen output, when viewed at arms length. Image content plays a huge part, which is why the number is a range, with x2 (150 LPI x2 = 300 PPI) being considered the upper limit. 

This brings me to the second common misconception, it's PPI – NOT DPI. Digital images are comprised of Pixels. The term is PPI, from the Pixels Per Inch.

If you are printing with an inkjet printer, then the stochastic (not halftone) screen isn't based on the old 300 PPI formula. Having 300 PPI can't hurt, however, it may mean that file sizes are needlessly excessive.

"photographs"... "prepared the file" -- more than one photo in the psb?

Which version of Ps?

editing to add: in my search of this community database, I found a few statements that where the file is stored can result in corruption of the file. Examples: cloud, network or external drive.

The conversations I have read do not have a "right answer," no solution offered.