Here’s what fixed it for me (verifying game etc didn’t work)
Open task manager and kill tos.exe
Go to your steam folder then to steamapps/common/TreeOfSavior/release/patch
If you see two files, one named “tos.exe” and one named “tos.exe.temp”
Delete the “tos.exe” file and rename “tos.exe.temp” to “tos.exe”
Most likely your antivirus is preventing the patcher from doing this operation.
Relaunch game, enjoy.