A memory debugger ran by my web host revealed this:
send(5, "\5\0\0\0\31\3\0\0\0", 9, MSG_DONTWAIT) = 9
send(5, "\204\0\0\0\26\n\t\t\tSELECT title, phrase_te"..., 136, MSG_DONTWAIT) = 136
poll([{fd=5, events=POLLIN|POLLERR|POLLHUP}], 1, 1471228928) = 1 ([{fd=5, revents=POLLIN}])
recv(5, "\f\0\0\1\0\4\0\0\0\2\0\1\0\0\0\0\27\0\0\2\3def\0\0\0\1?\0\f?"..., 129, MSG_DONTWAIT) = 129
poll([{fd=5, events=POLLIN|POLLERR|POLLHUP}], 1, 1471228928) = 1 ([{fd=5, revents=POLLIN}])
recv(5, "title\f?\0d\0\0\0\375\203P\0\0\0g\0\0\5\3def\27hobby"..., 169, MSG_DONTWAIT) = 134
send(5, "\20\0\0\0\27\4\0\0\0\0\1\0\0\0\0\1\375\0\0010", 20, MSG_DONTWAIT) = 20
poll([{fd=5, events=POLLIN|POLLERR|POLLHUP}], 1, 1471228928) = 1 ([{fd=5, revents=POLLIN}])
recv(5, "\1\0\0\1\2[\0\0\2\3def\27hobbymac_xen_forum"..., 35, MSG_DONTWAIT) = 35
poll([{fd=5, events=POLLIN|POLLERR|POLLHUP}], 1, 1471228928) = 1 ([{fd=5, revents=POLLIN}])
recv(5, "in\22xf_phrase_compiled\22xf_phrase_"..., 169, MSG_DONTWAIT) = 169
poll([{fd=5, events=POLLIN|POLLERR|POLLHUP}], 1, 1471228928) = 1 ([{fd=5, revents=POLLIN}])
recv(5, "\0\0\0\5\0\0\4\376\0\0\2\0R\0\0\5\0\0\31unexpected_er"..., 169, MSG_DONTWAIT) = 107
send(5, "\5\0\0\0\32\4\0\0\0", 9, MSG_DONTWAIT) = 9
poll([{fd=5, events=POLLIN|POLLERR|POLLHUP}], 1, 1471228928) = 1 ([{fd=5, revents=POLLIN}])
recv(5, "\7\0\0\1\0\0\0\2\0\0\0", 62, MSG_DONTWAIT) = 11
write(1, "An unexpected error occurred. Pl"..., 53An unexpected error occurred. Please try again later.) = 53
Note the last command.
send(5, "\204\0\0\0\26\n\t\t\tSELECT title, phrase_te"..., 136, MSG_DONTWAIT) = 136
It looks like the sql query generates an error somehow.
I would ask the dev if this helps him diagnose it, but he has refused to help.