This kind of access error is becoming quite frequent (Error code 500)
I too have been having this problem recently.
I've forwarded your report to the Support Team.
My (very personal - so possibly wrong) idea is that you tried to upload something during a spam attack and the server was too busy to handle everything. There are many, lately :( but the Team is working on that.
I've noticed that If the site is down with this kind of problem, accessing it using an anonymous browser window allows the access. I wonder why ...
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