Planner > Add Task > Add Attachment > Choose "Link" > Paste link including query string.
Expected result: Clickable link.
Actual result: Fails with "Please choose a valid URL".
It seems to be the ampersand that triggers it.
This is a showstopper if you want to include a link to a Service Desk Plus ticket, for example.
<rant>Planner development appears to have stalled - not saying you need to overdo it but it's hardly finished. Having Planner appear as a tab in Teams when it doesn't keep up with Teams is only going to get more frustrating for users. There's a character limit on the destription field - you saving disk space back there, Microsoft? There's a character limit on checklist items. </rant>
