Sending files with continous hex pattern via FTP causes corruption

I'm experiencing very difficult to diagnose problem, which causes file corruption on files I'm uploading. Corruption happens to many files which contain continuous hexadecimal pattern (like thousands of "FF FF FF").

Please have a look at the comparison of the same file in Hex view. The file I was trying to send is on the left and the same file after sending it via FTP is on the right:

As you can see, the file on the right has "00 00 00" instead of "FF FF FF".

In attempt to diagnose the cause I did the following:

• I made sure all my hardware has the latest firmware
• I tested my PCs against latest viruses
• I've been restarting router and modem several times
• I reset the router to factory settings
• Enabled and disabled firewall
• I tried various FTP clients
• I tried sending in Binary and ASCII modes, with and without MODE Z compression
• I tried various FTP servers
• I tried all of my machines (Win PC, Win laptop, Mac laptop even my android smartphone)
• I own NAS, so I asked my friends to download the files from it
• I excluded switch from my network
• I replaced the router
• I replaced the modem
• I replaced all Ethernet cables

None of the above helped apart resetting the modem.
Before I reset the modem the corruption ratio was 99% percent – literally every time I uploaded 100 files 98-99 of them were corrupted.
When it came to the step "restart the modem", the ratio dropped to 10%, so I thought it must be the modem and bought a new one, but after installing the new modem ratio of corrupted files remained at 10%.

I also created a hotspot on my mobile and connected my network to it, I could successfully send files without any corruption.
When I'm sending files which don't have the continuous hex pattern, the problem doesn't exist.

I'm on VDSL (BT Infinity), the Internet provider won't help, I think I exhausted all the options.

I'd appreciate any advise as I'm desperate for help.