Reproduced.

Win 7 Pro 64-bit Version: 5.1.0.0.alpha1+ (x64)
Build ID: 8b788891796ff0571f779cdbe8ce809c35c42754
TinderBox: Win-x86_64@62-TDF, Branch:MASTER, Time: 2015-07-02_23:09:27
Locale: fi-FI (fi_FI)

git bisect start 'latest' 'oldest'
# good: [0c30a2c797b249d0cd804cb71554946e2276b557] source-hash-45aaec8206182c16025cbcb20651ddbdf558b95d
git bisect good 0c30a2c797b249d0cd804cb71554946e2276b557
# good: [2ce02b2ce56f12b9fcb9efbd380596975a3a5686] source-hash-17d714eef491bda2512ba8012e5b3067ca19a5be
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# good: [40875247f0002056effdf6d2fbe43627691cd86c] source-hash-93f0b14458a618ad575cd446680e5c4aa7d87bdc
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git bisect good a79fa9e81bb9129d0382fdd38170054c720cf95d
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# bad: [136b8f86a09839b55e7c25e4cf82e439c496f3ce] 2015-07-07: source-hash-0251e61640b94094918406b33ee7b05564409feb
# good: [2b392af9c8f54629e3a3a98a8c92fa5af1c6722f] 2015-05-20: source-hash-90e2dabb8d0bb5382234be776c2ad0e2d5d9e224
git bisect start 'master' 'oldest'
# bad: [2b9d93baaf7341c57f25ff71fef3fcd2e10c6a73] 2015-06-13: source-hash-a9286c302e96258f32d13fb66aced25750e82cd3
git bisect bad 2b9d93baaf7341c57f25ff71fef3fcd2e10c6a73
# bad: [587d15ec717aed5d4abe657f3a96d4de29565bc7] 2015-06-01: source-hash-e61fcfea5c2a306e44a053976e921160e78917ac
git bisect bad 587d15ec717aed5d4abe657f3a96d4de29565bc7
# good: [991c3db554ae2f30bcc215ccbe090656d9970c02] 2015-05-26: source-hash-af75d7a4c99414fabbd31b9df590266d28574fb1
git bisect good 991c3db554ae2f30bcc215ccbe090656d9970c02
# bad: [04eea9ee476ef77555047447a98d1cc968f8a3c8] 2015-05-29: source-hash-ef2e135cfb8f107a154aaab9b54f2dfca0edf6a0
git bisect bad 04eea9ee476ef77555047447a98d1cc968f8a3c8
# bad: [7b138bcf32b52eccf484bd66d9ab5c70a7970805] 2015-05-28: source-hash-be01d68420086fc36ecf26b5f597ba7c6b29b369
git bisect bad 7b138bcf32b52eccf484bd66d9ab5c70a7970805
# good: [0c21dcaa08976385a72c7cd698a4474bcae7117a] 2015-05-27: source-hash-d56c1f7a20d8a95c88ea31c7f9000b8e270ab1ee
git bisect good 0c21dcaa08976385a72c7cd698a4474bcae7117a
# first bad commit: [7b138bcf32b52eccf484bd66d9ab5c70a7970805] 2015-05-28: source-hash-be01d68420086fc36ecf26b5f597ba7c6b29b369

This bug first appeared in this commit http://cgit.freedesktop.org/libreoffice/core/commit/?id=77c47fc79c1c8096c19a81a0d320e75b84857144

Assigning to myself.

Marek Doležel committed a patch related to this issue.
It has been pushed to "master":

http://cgit.freedesktop.org/libreoffice/core/commit/?id=0bd582834b46dbbc5037310d45bac8885e6f2a07

tdf#92582 Revert "tdf#91544: fix freezing of Data source window"

It will be available in 5.1.0.

The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
http://wiki.documentfoundation.org/Testing_Daily_Builds
Affected users are encouraged to test the fix and report feedback.

Thanks for your help and the fix!