I don't think its worth wasting your time for this issue, as it appears to have more to do with the tile server rather than your specific application.
The only thing worth checking is the sub-domain configuration
It is possible to configure OSM to use only one of the sub-domains, e.g. "a.tile.openstreetmap.org". This can lead to requests queuing up and eventually failing.
All sub-domains will be used automatically for Bing.
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