E200004: Could not convert '<...>' into a number

Suddenly, the following error started to occur in the repository when
checking out or updating:

    svn: E200004: Could not convert
'?\F6R?\EF?\F2?\9Bn1?\85?\9F?\A4?\AFm?\F5?\95?\8A
?\D4RJ?\80w3'h?\9A?\ED?\CC{!p?\B3?\C6?\C2!P?\DE?\EA?\F0?\C3

    ?\8D;?\E8?\E4 ?\D9

                      s<=4' into a number

Cleaning up and running update again seems to change files properly,
but the error keeps coming back every time. I noticed using `svnadmin
verify` that the error applies to most revisions just after a specific
one. `svnadmin dump` also fails with:

    * Error verifying repository metadata.
    <same 'convert' error as above>

The repository was created with 1.9.1, and clients use the same
TortoiseSVN client version. I updated both svnserve and client to
1.9.4 (r1740329) with no success. `svn ls` gives the same error for
the revisions that fail with dump. `svnadmin restore` runs within 1
second but does nothing that can fix this. The server log shows
nothing else than the usual 'get-lock open get-latest-rev reparent'
messages.

I searched the Web and this mailing list about it, but could not find
a solution. Any help is much appreciated.

Thanks.

--
Vinicius Massuchetto