The Wau Holland Foundation, which has reportedly transferred €750,000 ($1 million) in donations from the public to WikiLeaks, has been sent a second reminder to file its 2009 accounts.
"But this has nothing to do with WikiLeaks," Michael Conrad, a spokesman for tax authorities in the western state of Hesse, told new agency AFP, saying the reminder was nothing out of the ordinary.
"We haven't got anyone in our crosshairs, we don't have any crosshairs."
WikiLeaks has found itself under growing international pressure following its release of secret US documents to several news organisations, with its donation pipelines slowly being cut off.
MasterCard Worldwide on Monday stopped funnelling payments to the organisation, and on Friday US-based online payment service PayPal blocked financial transfers.
The Swiss Post Office's banking arm shut down one of WikiLeaks founder Julian Assange's bank accounts on Monday, saying he had provided false information in his application.
PayPal has also shut down the Wau Holland Foundation's account, the foundation said, saying PayPal had told it it was "in violation of PayPal's Acceptable Use Policy regarding... financial support to WikiLeaks."
Australian national Assange, 39, was arrested in London on Tuesday. He is wanted in Sweden for questioning on suspicion of crimes including rape.
The Wau Holland Foundation, named after a "data philosopher" and co-founder of German hackers' collective the Chaos Computer Club who died in 2001, called on its website for people to continue supporting WikiLeaks.
It said that passing on donations, which are tax deductible to those giving money, to Wikileaks was lawful in Germany, and that bank transfers were still possible.
If the foundation fails to file its accounts, the authorities will compile them on their behalf — a service which the foundation will have to pay for, Conrad added.