This a fragment of page.html that had the error:

<b i18n:translate="">Monthly Maintenance</b><br>

    <a href="#"
       tal:attributes="href python:request.indexargs_url('issue', {
      '@sort': 'assignedto_multi,-activity',
      '@group': 'target_date',
      '@filter': 'keyword,status',
      '@columns': 'id,assignedto_multi,activity,title,status,priority,source',
      '@search_text': '',
      'keyword': '5',
      'status':'-1,1,2,3,4,9',
      '@pagesize':'300',
      '@startwith':'0',
      '@dispname': i18n.gettext('Monthly Maintenance - current'),
     })"
       i18n:translate="">Current maintenance</a>
---------------
This is what I changed it to which works fine:

<b i18n:translate="">Monthly Maintenance</b><br>

    <a href="#"
       tal:attributes="href python:request.indexargs_url('issue', {
      '@sort': '-activity',
      '@group': 'target_date',
      '@filter': 'keyword,status',
      '@columns': 'id,assignedto_multi,activity,title,status,priority,source',
      '@search_text': '',
      'keyword': '5',
      'status':'-1,1,2,3,4,9',
      '@pagesize':'300',
      '@startwith':'0',
      '@dispname': i18n.gettext('Monthly Maintenance - current'),
     })"
       i18n:translate="">Current maintenance</a>

The only difference is @sort now only has one key. Previously it had assignedto, 
which was converted to assignedto_multi which a MultiLink version of assignedto. 
I'm not sure why having only one sort key works, and two fails.

If you need the page.html file instead of these snippets let me know. That file 
contains usernames that I'll have to redact. You probably need the schema.py file 
too.