Use a better method than default printer default paper size to determine default page size.

test.mscz has a very stange page format, 99,82 mm * 23,88 mm and also strange page borders, 0.00mm on the 'inner' page border (left for odd, right for even)
As it works for that other file there's no blocker at all, not even critical

EnRundrejse.mscz works and looks OK, creating all parts, doers generate 3, whereas the score has hone 2, piano 1 and piano 2, those 2 do generated and look OK, but a third gets gegerated 'stemme' that shows the behavoir of text.mscz.
Actually that third one existed already. When, how, why dod ypou generate it? It seems to be for an instrument that isn't even part of the score?

No big deal: just delete it. Or use Format > Page settings > Reset to defaults

Came up too in https://musescore.org/de/node/316809

That test score looks pretty strange internally:

    <Style>
      <pageWidth>3.93</pageWidth>
      <pageHeight>0.94</pageHeight>
      <enableVerticalSpread>1</enableVerticalSpread>
      <Spatium>1.74978</Spatium>
      </Style>

Bogus/strange width and height, no page borders set at all, so while 15 mm either side should be the default, those 0 mm that we see when opening it, is not written in the score

Do you perhaps have a custom MSS set as your default score style in Edit / Preferences / Score? Or have a very unusual default printer or a strange default paper size specified on your system?

That indeed might explain this

FWIW, it shouldn 't be that hard to update an MSS file, depending on what you actually want to accomplish. Could be as simple as changing the version number form 3.01 to 3.02. Or, try loading it into a score, then immediately doing Format / Save Style. Probably best to ask for help on the Support forum and describe what you are trying to do in more detail.

In your .mss file I see:

  <Style>
    <pageWidth>8.27</pageWidth>
    <pageHeight>11.69</pageHeight>
    <pagePrintableWidth>7.4826</pagePrintableWidth>
    <pageEvenLeftMargin>0.393701</pageEvenLeftMargin>
    <pageOddLeftMargin>0.393701</pageOddLeftMargin>
    <pageEvenTopMargin>0.393701</pageEvenTopMargin>
    <pageEvenBottomMargin>0.787402</pageEvenBottomMargin>
    <pageOddTopMargin>0.393701</pageOddTopMargin>
    <pageOddBottomMargin>0.787402</pageOddBottomMargin>
    <pageTwosided>1</pageTwosided>
...

Very different from that of the test score

No, definitely don't overwrite anything in that folder. It's not supported, and it would get clobbered on the next update anyhow. If you have a custom file you want to be your default for all scores regardless of instrumentation (and thus isn't better handled as a template), you should specify it as the default in Edit / Preferences / Score. I assume that's what you are doing, but again, we'd want to understand more about what you are trying to accomplish here in order to advise the best way to update this for 3.6.

But do consider also the possibility that what you are trying to do is much more easily and effectively accomplished through the use of templates - they are more powerful and require no special options to use, just save a sample score to your Templates folder and its automatically available for use when creating new scores.

I have the same Problem. I was working with MS weeks ago. Today Update from 3.5. to 3.6.0 and the mentioned error appears with new document. Also if I start MS with default a new document. I tried different versions of MS to 3.5.x but same issue. I deleted folders AppData/Local/ and Roaming. I de- and installed MS, no change!
Why create MS a new document with customized settings and not with A4 as default setting?
My printer settings are A4. Also Resets dont help!
No custom files at all!

If you're saying you have the same problem, can you please attach the specific MSS file in question, and precise steps to reproduce the issue?

maybe that is no longer necessary :)
In my case the wrong printer was set as default printer. Its a label printer und the settings are not A4 of course und in landscape format. So I chose my normal printer as default printer and et voila, a new document is displayed correctly. I hope this is the solution for everyone who has the problem

But anyway, where can I find this MSS file?

this is my thread
https://musescore.org/de/node/316809#comment-1059154

If so this is completly unrelated to 3.6. At least for @Bbflat it is, had that issue with 3.5.0 too (after a fallback to that from 3.6, including a factory reset).
(There's an mss file attached further up)

Check also your default printer and its default paper format (in the computer with the issue, compare to the one without)
If @Bbflat's thery is right, this is not a regression at all

A workaround ist not a (good) solution ;)

Indeed not. But better than nothing ;-)

This taking the default paper format from default printer is something that should get changed anyhow, as it is a constant source of problems, like with temporarily unreachable network printers (see #150601: MuseScore does not open when a network printer is not reachable).
It should just be asked on 1st start and then stored in preferences, see #282736: MuseScore 3 does not set correct paper size in Linux

To use a template, don't save it to your Styles folder- MuseScore won't find it there and you won't be able to use it. you need to save the template (an actual score - MSCZ or MSCX file) to your Templates folder. Then it will be available for selection in the list of templates the next time you create a new score.

why I didn't know that the wrong printer was selected as the default printer:
1. I almost never print out sheets
2. setting in windwos: windows manages standard printers ;)

So, is there still some aspect of this that is not just a known / expected effect of having a bad default printer setup or else a bad MSS file specified as your default? If so, could someone please post precise steps to reproduce that?

So in other words, it's behaving exactly as designed - taking the default page size from your printer defaults. It's just that you are suggesting we use a better method. I agree :-)

Yep. A Major Suggestion ;-)