Bitcoin rival ethereum plunged Monday despite a major exchange's efforts to shore up confidence by announcing a credit for customers who lost money during last week's flash crash.

Ethereum, also known as ether, traded 20.9 percent lower at $239.63 Monday afternoon, according to CoinDesk.

Last Wednesday, ethereum briefly plunged in a flash crash from above $300 to 10 cents on Coinbase's GDAX exchange. On Friday, the exchange said it would credit customers who "experienced a margin call or stop loss order" on GDAX during the flash crash.

GDAX reported Monday afternoon, ET, a temporary halt in trading for all products, which was resolved within an hour.

Ethereum one-day performance

Source: TradingView, Coinbase

Ethereum is still up about 2 percent for June, and 2,500 percent for the year, according to CoinDesk data. At its peak, ethereum had surged more than 4,000 percent for the year.

An unsubstantiated rumor Sunday that ethereum's founder Vitalik Buterin had died also reportedly hit ethereum's price for a brief period. Ethereum fell from around $329 to a low of $284 Sunday before recovering, according to CoinDesk. Buterin tweeted a picture of himself Sunday evening, ET, to dispel the rumor.

William Mougayar, author of "The Business Blockchain: Promise, Practice, and Application of the Next Internet Technology," said he did not believe the rumor was the main factor behind ethereum's decline.

"We need to see a full 24 hour cycle in order to fully assess what is actually happening," he said in an email to CNBC. "These cryptocurrencies are traded globally, especially with a large amount of activity coming from China, so I'm waiting to see what happens around 6PM EST this evening."

Digital currency investors also pointed to the reversal of last week's exuberance stemming from the launch of several ethereum-based projects, which ended up clogging the network. The inability of ethereum to handle the orders raised worries that digital currency may soon face a debate over an upgrade model, similar to a heated controversy over bitcoin's future that has weighed on that currency.

Two different methods for upgrading bitcoin, SegWit2x and BIP148, are set to go into effect July 21 and Aug. 1, respectively. Initially the systems were incompatible, threatening to split bitcoin in two and likely reducing its value. However, a developer announced earlier this month a way to make the two methods compatible, helping bitcoin recover from lows of the month.

Bitcoin traded nearly 5 percent lower Monday around $2,452, still up about 5 percent for the month and 153 percent higher for the year, according to CoinDesk.

In a blog post Monday, digital currency investor and Union Square Ventures partner Fred Wilson said the decline in ethereum was profit-taking after a massive run higher in the last six months.

"My gut says we are headed for a selloff in the crypto sector," he said in the post. But he said he remains optimistic about the future of cryptocurrencies over the next five to 10 years.

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Ethereum is crashing by 20% right now after confidence in bitcoin rival shaken - CNBC