Frequent error in the logs

Can anybody explain, where this error comes from?

Logger: homeassistant.components.mobile_app.notify
Source: components/mobile_app/
Integration: Mobile App (documentation, issues)
First occurred: 15. Juli 2021, 19:00:06 (1 occurrences)
Last logged: 15. Juli 2021, 19:00:06

Error sending notification to ContentTypeError(RequestInfo(url=URL(''), method='POST', headers=<CIMultiDictProxy('Host': '', 'User-Agent': 'HomeAssistant/2021.6.4 aiohttp/3.7.4.post0 Python/3.8', 'Accept': '*/*', 'Accept-Encoding': 'gzip, deflate', 'Content-Length': '337', 'Content-Type': 'application/json')>, real_url=URL('')), (), message='Attempt to decode JSON with unexpected mimetype: text/plain; charset=utf-8', headers=<CIMultiDictProxy('Connection': 'keep-alive', 'Content-Length': '36', 'Content-Type': 'text/plain; charset=utf-8', 'Server': 'Google Frontend', 'X-Cloud-Trace-Context': '0d59d3669e591148c402ab2a505a0a11', 'X-Content-Type-Options': 'nosniff', 'Accept-Ranges': 'bytes', 'Date': 'Thu, 15 Jul 2021 17:00:06 GMT', 'X-Served-By': 'cache-hhn4081-HHN', 'X-Cache': 'MISS', 'X-Cache-Hits': '0', 'X-Timer': 'S1626368400.065845,VS0,VE6135', 'Vary': 'x-fh-requested-host, accept-encoding')>)

Well, the error comes from here and is caused by the server ( returning an incorrect Content-Type HTTP header (text/plain; charset=utf-8 instead of application/json).
However, I have no idea why this happens.