rendering – Unity – Weird Shadow Render Error

Using Unity 2018.2.14 / 15 I recently worked on a small project and I encountered a very strange problem. It's hard to describe so here's a picture:

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Initially, I thought that this could be a problem with my shadow / lighting settings (I think that sounds a lot like that), but I found no settings having a direct impact on the lines.
Ignoring this is unfortunately not an option and I do not know of any workaround …

Do you have an idea of โ€‹โ€‹what is going on / what can I do to prevent these lines from appearing?

Thanks in advance! ๐Ÿ™‚