The message for svnadmin dump is svnadmin: malformed file. However svnadmin verify does not report any errors at all. I searched through a few backups and did a binary comparison of the repositories with winmerge (which is binary and actually compares the contents, not just the file dates). In my backup that was made 1 day after the revision was created I did not find any differences. So I don't think the file became corrupted on disk over time. So what can I do now? Btw, this is certainly a bug in SVN because verfiy does not find the error.
I have seen the issue you have described, because I use FSFS rather than BDB as long as I can identify where the corruption has occurred I can edit the text files to correct the inconsistency, this has worked for
svnadmin verify positive and negative corruption.
I host Subversion through Apache with mod_dav_svn, more than once it has been suggested that it could be an Apache threading issue.