Since few of my friends have similar issue, I’ll post a possible fix here.
Go Steam > Library, in Library tab right click Tree of Savior then go Properties > Local Files > Verify Integrity of Game Cache.
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Thank you! This fixed it right away.
For reference, this is what the error dialog looked like:
I hope this helps anyone having the same problem to identify this as the proper fix.
OMG!! Thanks for the tech advice, Silvestris. It immediately fixed the update. You are awesome.