This may be temporary (at their end) or long term.
My best guesses are the usual reasons that I've already provided.
To expand the "liquidity" issue, even famous Exchanges sometimes experience that issue when there's too much volume flowing in their payment channels.
Their channels' amount simply cannot accommodate the requested amount.
To expand the "routing" issue, it could be a combination of your invoice lacking the necessary "route hint" (issue at your end)
and/or their node simply cannot find a route to your channel (issue at either end).
Switching to "Gossip" and waiting for full lightning network connection could get your Electrum better routing but it wont fix the issue if it's the Exchange's channel's fault.