This bug is fixed in version 3.13.
Please update your TeX system to obtain current (and matching) versions of biblatex and Biber.
The answer below is kept for historical interest and background info.
This is a very elusive bug in biblatex. After discussions in https://komascript.de/node/2242, the KOMA-Script developer reported the issue at https://github.com/plk/biblatex/issues/894 and suggested a possible fix that was added to biblatex with https://github.com/plk/biblatex/commit/26c2072829d78447a9bb1931bd22662b34a19e18.
The issue is resolved in biblatex v3.13.
The bug only occurred if the biblatex language module (the .lbx) for <language_1> inherits from <language_0> in a particular way and <language_0> is not loaded as a document language. In these cases biblatex would add an undefined macro to \extras<language_0>. Since <language_0> is not loaded, this macro would normally not be used and so this would not cause any issues. But titlepage uses \providecaptionname which hooks into \extras<language> in such a way that the undefined commands caused a problem.
In the specific case of the MWE british inherits from english, but english is not loaded. Still, biblatex writes undefined commands to \extrasenglish and \providecaptionname trips up.
The problem does not occur with ngerman because the inheritance setup between ngerman and german is slightly different, which means that the macros that end up undefined in the british case are actually defined here.
Until the version with the fix is released I can offer two workarounds
Just load english as well as british. (Be sure to get the load order/main language right.)
Incorporate the suggested fix into your preamble. Note that the fix should be removed from your preamble when a new version of biblatex is released. Hence the code block below will produce an error with upcoming biblatex releases.
\makeatletter
\@ifpackagelater{biblatex}{2019/05/28}
{\blx@error
{'\string\blx@maplang' fix no longer needed. Remove it}
{Remove the redefinition of '\string\blx@maplang' from your preamble}}
{\def\blx@maplang#1#2{%
\csgappto{extras#1}{%
\blx@resetpunct
\csuse{abx@extras@#2}%
\csuse{abx@strings@#2}}%
\csgappto{noextras#1}{%
\blx@resetpunct
\csuse{abx@noextras@#2}}}}
\makeatother
fullsizetitleis undefined. So your code does not work if you do not load packagetitlepage. – esdd May 27 '19 at 14:23babel, too. But packagetitlepageis needed. – esdd May 27 '19 at 14:40titlepage-11.tds.zipfrom https://komascript.de/node/1213 and unpacked thetex/latex/titlepagefolder into an empty directory. In that directory I then created amwe.texwith the contents of your last code example. Then I ran LaTeX on that document and got no errors. Everything shows as expected. I added\listfilesto the beginning of the file so you can compare versions more easily. You can find my complete.logand the.texat https://gist.github.com/moewew/2eb75a9e9fd48ea19b314674d571bd52. – moewe May 27 '19 at 18:29.texwith\listfilesand upload the complete.logfile to a text-sharing website such as https://pastebin.com/, https://gist.github.com/, https://hastebin.com/ ... – moewe May 27 '19 at 18:30.logis not complete and (maybe more importantly) it is not the.logof the MWE you posted here or on the KOMA-Script forum. Your.logloadsscrreprt.cls, but the MWEs all usescrbook.cls. There are a few other differences in load order as well. Please run the shown MWE and upload the.logfile for the MWE. If the MWE does not reproduce the issue change it so that it does. – moewe May 27 '19 at 19:10