Firstly, can you please contact wpengine and ask them to take a look at the issue. I believe this is a server issue not related to ACF.
Whilst you wait for their reply, can you answer the follow q’s?
1. Has the options page eve worked on this install?
2. Do any other areas of the site cause a similar server error?
3. Have you attempted to disable all plugins to test?
4. Have you attempted to swap to default theme?
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