So I went to Samut Prakan immigration today and spoke to 2 different officials. They said that the reason that my report was rejected with the reason "Incorrect entry date upon entry into the kingdom/ Incorrect visa expiration date." was not in fact for those reasons but because I had left it too late (2 days before 90-day deadline). They also said it was nothing to do with it being my first report after leaving the country. The first official, who had loads of stripes down her arms, said submitting a 90 day report online must be done exactly 15 days before the 90-day deadline, no sooner, no later, even if that's a weekend. The second official was a bit more relaxed about the exact day but said that the system is not working well and is automatically rejecting lots of applications. He said the sooner you do it before the deadline (up to 15 days before), the better. He said next time, if it gets rejected, call him on 0631871178 to see if he can get it approved without me having to go there in person. I guess that's a Samut Prakan number so probably no use calling it if you're in another province.