Recently, some people have been getting this error message: Download Failed: Cannot read file. c25. When (GS)Rattler told me what he had to do to get around the error message, I remembered encountering this problem years ago (EP4). I am not even going to pretend that I understand the particulars on how anti-malware programs work. However, occasionally over the years, antivirus programs, firewalls (including Windows Defender), anti-malware, ad blockers, etc. have detected Shaiya's updater as a malicious program. While this is not the case, it is not possible to update the game because these programs have blocked access to it. I wish to thank (GS)Rattler for a solution. I passed this solution to someone and they were able to finally login to the game.
Because the updater.exe is being detected as malware, both Shaiya and updater.exe need to be unblocked by going into the settings all of your anti-virus, firewall (including Windows Defender), and any other anti-malware programs.
Then turn off all anti-virus, firewall (including Windows Defender), anti-malware programs. etc.
Run the Shaiya updater then turn all those programs back on.
The c25 error message along with other Troubleshooting answers can be found in Technical Troubleshooting FAQs.