There are messages all over the place to the effect that the OSM servers will be in read-only mode until about 2012-04-03 (today). This is understandable but why do I keep getting Internal Server Error when JOSM tries to download ? This has been happening ever since the server supposedly went RO. I understand I won’t be able to upload stuff, but why can’t I download stuff ?
There is a lack of clear information which API operations precisely are available in the so-called read-only mode. But it seems that map calls, as used by the data layer and (afaik) JOSM, are not available, despite being a read operation.
You could try downloading from a data mirror, e.g. using the “mirrored download” plugin:
The way the map call is programmed, it needs write access to the database to write temporary tables that it uses to assemble the results. As the database itself is in read-only mode, these temporary tables can’t be created and the map call fails.
Although an Internal Server Error isn’t nice and a better error message might be helpful, “fixing” this isn’t particularly high on the priority list given how seldom read-only mode is used. Furthermore, it might actually be a good thing for standard JOSM and Potlatch downloads not to succeed, as it is even more confusing and frustrating if the uploads fail after having put in a lot of effort into mapping because one didn’t know about the read-only situation.
As Tordanik said, you can use the mirrored download plug-in though and hopefully the API should be back soonish.