Confirmed. Downloaded from http://www.opentaal.org/projecten/synoniemen.html and took the 3.x version. Installs fine in LO and AOO 4.1.2, runs in AOO (after some hassles) but refuses to work in LO 5.2.2. Might be a bug in LO, might be something in the file that’s no longer supported.
Filed bug report here: https://bugs.documentfoundation.org/show_bug.cgi?id=103045
Answered by Urmas on the bug list:
"The Dictionaries.xcu contains
malformed property key for supported
locales, so it doesn’t work.
(To fix it manually, extract the said
file from the OXT archive, replace all
4 nl_XX tags with a
single one, separating their values
with spaces, like “nl_NL nl_BE …”,
then re-add the corrected file back."