In the Limesurvey administration screen, go to "Edit Labels".
Create two label sets with three labels each.
In the "Label Sets List" screen, click on "Export Multiple Label Sets".

Version A:

4A) In the "Export Multiple Label Sets" screen, select multiple label sets. The "Export" button changes from grey to green.
5A) Click on export. The .lsl file exports successfully. However, the Limesurvey screen remains in loading mode (greyed out with a spinning wheel). It is impossible to exit this mode directly. However, by refreshing the page, Limesurvey returns to normal operation.
There is no specific error message.

Version B:

4B) Select the labels, and the "Export" button changes from grey to green. Deselect the labels. The button does not turn back to grey; it remains active. 5B) Click on export. A dialog box appears stating "You must select at least one label set," which is normal. After clicking OK, there is no export, which is expected, but the screen returns to the grey loading state.