"Culture is not supported" error when trying to confirm a segment

I couple of months ago I installed Studio 2017 Freelance (SR1 - 14.1.10015.44945). Today, I get "Culture is not supported - Parameter name: name - ES-419 is an invalid culture identifier" error message when trying to confirm a segment. This happens on two different files of two different projects. I restarted my PC, removed the project from the project list and opened it again, and finally deleted the project folder and opened the package again, but none of these worked. Source language is en-US and target language is el-GR, and my OS is Windows 8.

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