Deleting the .umap and starting still resulted in a crash i looked through the log and found what i think to be the problem; this is an excerpt from the log:
“[2015.07.01-09.14.47:343][ 0]LogThreadingWindows:Error: Runnable thread FAssetDataGatherer crashed.
[2015.07.01-09.14.47:343][ 0]LogWindows: === Critical error: ===
Assertion failed: InPos<=Bytes.Num() [File:D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.8\Engine\Source\Runtime\Core\Public\Serialization\Archive.h] [Line: 171]
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Crash in runnable thread FAssetDataGatherer”
It seems that the asset loader is crashing for some reason: possibly more corrupt files?