I started the "Incident in Yellow Forest" Exploration with 4 eggs in my party, then filled it with 2 more while getting through the first two tasks. On the third task, a Scyther was supposed to get put into my party... and it was.
I didn't really think anything of it until I tried to move my Underground Pokemon: error. Mobile Walker: error.
The error said something about insufficent party members with a 7 in parentheses.
I ended the Exploration and it fixed itself after a few minutes, but I figured I'd put this here just in case.
This has been a known issue for almost the entire decade GPX has existed. As far as I know it's caused by lag causing the site to progress the exploration forward without realizing you don't have the open party slot you should be required to have. (The mobile site also has an issue where it doesn't even check for an open party slot at all, and just lets you progress forward regardless.) As you discovered, the solution is to either end the task or take things out of your party until you're down to the correct amount.
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