Nearly 300 Maryland residents who thought they had successfully registered to vote using the state’s new online system actually had not, and the glitch will force them to file provisional ballots on Election Day.

A separate error has resulted in Maryland elections officials mailing to absentee voters in the Washington area at least 15 ballots missing a page to vote on the state’s high-profile ballot measures, including same-sex marriage, an expansion of gambling and in-state tuition breaks for illegal immigrants.

Most of the voters affected by the online registration glitch may not know yet. They may only learn of the problem when they reach polling places Nov. 6 and are forced to fill out provisional ballots, which are not counted until after the election.

“It seems like it lessens my vote . . . it sort of seems like disenfranchisement,” said Seth Marcusa, who is stationed in Maryland on active duty with the Navy. Marcusa and his wife registered together online last month, but when she later received a voter registration card in the mail and he did not, Marcusa began making calls to local elections officials.

He ended up at the Maryland State Board of Elections, where he said he was told a glitch had incorrectly “attached his information to another voter’s name.”

“They could see what I did online, but I was told there was nothing I could do,” he said. “I do wonder how many people will go to vote on Election Day and be turned away because they’re not registered the way they thought they were.”

Ross Goldstein, deputy administrator of the elections board, said the state has gotten to the bottom of the issue. He said that he could not yet explain the underlying technical glitch but that fewer than 300 voters are affected. Because their files were not corrected until after the state’s registration deadline, however, they must use provisional ballots.

The state’s new online voter registration system — which is locked to further changes until after Election Day — has faced withering criticism from cybersecurity experts. They say it has left voters’ data vulnerable to tampering. The new system links residents’ voter records with their driver’s license numbers. The latter can be determined online by anyone who knows a voter’s full name and birth date.

Alisha Alexander, the elections administrator in Prince George’s County, said officials there have received complaints similar to the one levied by Marcusa. “They thought they registered online, but were not in our system,” she said.

Goldstein also said the elections board is still investigating why at least 15 absentee voters in Montgomery and Prince George’s counties have reported receiving ballots missing a second page that contains boxes to vote on key ballot questions.

In Prince George’s, where a razor-thin margin could decide the fate of a plan for an $800 million casino at National Harbor, the problem surfaced about a week and a half ago.

Alisha Alexander, the county’s elections administrator, said a couple drove to the county’s elections office to show ballots containing only the first page with boxes for federal races.

Goldstein said the elections board is still searching for an explanation and added that there appears to be no common pattern, party affiliation or otherwise, among the 15 who have said they received incomplete ballots.

Gov. Martin O’Malley (D) issued a statement Wednesday suggesting that the number identified so far was the extent of the problem, but he encouraged anyone with an absentee ballot to check to make certain it is complete.

Alexander said a third man in Prince George’s has since come forward missing a page, the remainder of the incomplete ballots have been found in Montgomery County

“They don’t know how much it could affect,” Alexander said of state elections’ officials understanding of the extent of the problem, “they’re getting that information from us.”