User can't submit bulk approvals from his/her Work List any more.
In this specific situation, as it tured out, we saw that the approver in question had over 150 work items in their worklist, which is the SQLQueryLimit.
We also noticed that the approvals with the problem were actually to approved by the same user twice.
Because the Worklist is limited to 150 items, some of the tasks in progress are actually waiting for this same users approval, but cannot be seen as they are not in the 150 work items displayed to them.
As the SQLQueryLimit cannot be increase due to resource constraints, the only option is/was to only approval first stage approvals using the bulk approval at first.
Once all of those approvals had been completed and no further first stage approvals appear, you should approve the second stage approvals.