No other ideas? :/

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I would definitely contact UBB support, and see if they can figure out what's going on. We've exhausted our resources; without input from the developers, this issue will not be able to be resolved as we haven't gotten any other complaints regarding PHP, GD2, or ImageMagick on your host. We can try migrating you to a different host if you'd like.

GangsterBB.NET (Ver. 7.6.1.1)
PHP Version 5.6.40 / MySQL 5.7.23-23 (was 5.6.41-84.1) / Apache 2.4.54
2007 Content Rulez Contest - Hon Mention
UBB.classic 6.7.2 - RIP