I would like to test some data before life upload. That should be possible via https://master.apis.dev.openstreetmap.org
If I understand correctly I need to make a new account. I tried that but I have to activate this via an email but I do not receive this email. Any suggestions?
Also … if I want to use JOSM for upload … what do I need to change in order to connect to this DB?
Perhaps request the password or check the spam folder?
https://master.apis.dev.openstreetmap.org/user/forgot-password
Change the OSM-server-address in the JOSM settings to:
https://master.apis.dev.openstreetmap.org/api
And of course enter the new login data.
Just in case: the dev instance does not have a copy of the current data, if your upload is making any changes to “existing” data it is going to fail.
It just seems that it takes a little while before the email arrives . So I just should not be so impatient 
Thanks. I want to upload new data right after I delete existing OSM (dev) data so that I can see what the result is. The new data should replace existing polygons with mainly landuse and natural keys. The new data is from a (mostly) more accurate NL public (osm compatible) dataset. It is just a test to see if it works and if so … see what the next steps can be. I am aware of OSM import rules.
Does anyone know if the DEV data is replaced/refreshed by (a copy of) production data every now and then?
Yes and no it isn’t. There is literally just random test data from whatever people have uploaded (and it hasn’t been reset since ages, if ever).
It’s possible to copy a small corner of live to dev - I suspect you can still do that by copying to a new layer in JOSM, or at worst manually editing a .osm file to contain all new stuff.
Just for whoever might run into this same situation. I found out that in my test area there was no data on dev server. I downloaded data from production server to JOSM. After that I changed server to DEV in JOSM settings and uploaded to DEV. So I can now start testing on DEV server.
NB
In my first attempts I was restricted because of rate limiting. I sent an email to the DWG so that they could hopefully change these limitation settings on my DEV account. I have not heard anything yet but settings might have been changed because I can now upload a relatively big dataset to DEV server.
The ticket hasn’t been picked up by anyone yet. It was briiefly mentioned on IRC, but that seems to be because the account name you quoted in the ticket does not exist. For the avoidance of confusion your dev account seems to be https://api06.dev.openstreetmap.org/user/PeeWee32_test . That account is not currently an importer but I have asked the admin in IRC to enable it.
Edit 17:25: That’s now been done.
An other question. After testing a big set of data I want to delete the entire set from dev server so that I can replace with a better version. I currently do this with JOSM by downloading tiny bounding boxes and then delete (CRTL+A) and upload. This way I get many conflicts (related to relations) and is far from ideal. Is there a better way to delete a big set from dev server?
are you downloading also relations? JOSM should work fine here
I wouldn’t say it was better, but the perl scripts include an explicit object delete option. If for whatever reason JOSM isn’t an option. Just make sure you delete from the dev and not the live server 
It is most likely me . I’ve uploaded quite some data to dev. Currently in the Rotterdam area. (but it might all be gone soon because I want to delete this in an easy way) . Because I can not download all the data to JOSM in one go I do this step by step. I download smal area in JSOM (version19342), then select all (ctrl+A) en press delete and then upload. Every now and then when deleting data from changeset I get errors/warnings regarding relations that I am about to delete and also about primitives.
When trying to upload to server I get this error.
It may very well be that I should follow some other procedure to delete data from dev server but I have no clue (yet). I will take a look at the suggestion of @TrickyFoxy .
how exactly you download data?
this warns you that you have not downloaded referenced relations
I just press the green download buttons.
And after download I do see relations.
I will update/reinstall JOSM and see what happens.
I’ve just updated JOSM (using webstart) but I am still having the same issues. After my attempt it seems that (most of the) ways are gone but the nodes are still there. Could you test in the Rotterdam area if you can remove data without any issues?
seems to be working fine: Changesets by test_mod_account | OpenStreetMap for example Changeset: 410962 | OpenStreetMap
initially I hit conflict (though on way, not relation) but that was because I deleted node outside fully downloaded area (marked with hatching in JOSM)
Thanks for testing. 
I have no clue why these strange things are happening to me/ my JOSM version but I succeeded in removing all my data and that was the goal.
