ExtCal sends the value of variable $lang to the template in calendar.php. This apparently gives problems when using Dutch language.
When we change the variable to e.g. $lan everything works fine.
<tr style="text-align:center;" class="head">
<td>Monday</td>
<td>Tuesday</td>
<td>Wednesday</td>
<td>Thursday</td>
<td>Friday</td>
<td>Saturday</td>
<td>Sunday</td>
</tr>
<tr>
<th style="vertical-align:middle;">35</th>
<tr style="text-align:center;" class="head">
</tr>
<tr>
<th style="vertical-align:middle;">35</th>