I'm having the same problem. Not able to upgrade tvheadend anymore (nor install on fresh Ubuntu VB).
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https://dl.bintray.com/tvheadend/deb xenial InRelease
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https://dl.bintray.com/tvheadend/deb xenial Release
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https://dl.bintray.com/tvheadend/deb xenial/stable amd64 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable i386 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable all Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en_US
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en
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https://dl.bintray.com/tvheadend/deb xenial/stable amd64 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable i386 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable all Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en_US
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en
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https://dl.bintray.com/tvheadend/deb xenial/stable amd64 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable i386 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable all Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en_US
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en
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https://dl.bintray.com/tvheadend/deb xenial/stable amd64 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable i386 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable all Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en_US
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en
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https://dl.bintray.com/tvheadend/deb xenial/stable amd64 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable i386 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable all Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en_US
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en
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https://dl.bintray.com/tvheadend/deb xenial/stable amd64 Packages
404 Not Found
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https://dl.bintray.com/tvheadend/deb xenial/stable i386 Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable all Packages
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en_US
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https://dl.bintray.com/tvheadend/deb xenial/stable Translation-en
Fetched 102 kB in 2s (43.5 kB/s)
Reading package lists... Done
W: The repository '
https://dl.bintray.com/tvheadend/deb xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch
https://dl.bintray.com/tvheadend/deb/dists/xenial/stable/binary-amd64/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
tvheadend@tvheadend:~$ sudo apt-get install tvheadend
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package tvheadend