File 'cache/I/9/*****.header' has a version mismatch. File had version: xxxx

[Mar Imp <marimpimp@xxxxxxxxx>]

Hi community!

I'm running Apache 2.4.4 on a Windows 2012 Standard Server. It is a caching reverse proxy.

I get a lot of cache_disk errors in the logfiles:
[log]
[Thu Jun 12 13:25:43.147301 2014] [cache_disk:error] [pid 33044:tid 2544] [client X.X.X.X:10178] AH00705: File 'cache/I/9/4JRTaMjqqNUQo7uX01sg.header' has a version mismatch. File had version: 107638736., referer: https://***********
[Thu Jun 12 13:25:43.173319 2014] [cache_disk:error] [pid 33044:tid 2536] [client X.X.X.X:10179] AH00705: File 'cache/9/w/aAdfdQot462I@RksGCEg.header' has a version mismatch. File had version: 116356112., referer: https://***********
[Thu Jun 12 13:25:43.177323 2014] [cache_disk:error] [pid 33044:tid 2544] [client X.X.X.X:10178] AH00705: File 'cache/s/V/qV02LgwwtjEJyxKaznPA.header' has a version mismatch. File had version: 107687984., referer: https://***********
[Thu Jun 12 13:25:43.181327 2014] [cache_disk:error] [pid 33044:tid 2712] [client X.X.X.X:10181] AH00705: File 'cache/C/5/R3wCvvPQ8B3uW_gS9vUA.header' has a version mismatch. File had version: 107854032., referer: https://***********
[Thu Jun 12 13:25:43.187331 2014] [cache_disk:error] [pid 33044:tid 3044] [client X.X.X.X:10180] AH00705: File 'cache/0/i/FKYwLK7sYyuUdX5vMWZA.header' has a version mismatch. File had version: 115647536., referer: https://***********
[Thu Jun 12 13:25:43.195348 2014] [cache_disk:error] [pid 33044:tid 2756] [client X.X.X.X:10182] AH00705: File 'cache/J/B/FxxEDkelxUMPh8FD9bqw.header' has a version mismatch. File had version: 111061968., referer: https://***********
[Thu Jun 12 13:25:43.206350 2014] [cache_disk:error] [pid 33044:tid 2544] [client X.X.X.X:10178] AH00705: File 'cache/p/7/PNGH8mmusYQpevikBZCg.header' has a version mismatch. File had version: 107589488., referer: https://***********
[Thu Jun 12 13:25:43.239381 2014] [cache_disk:error] [pid 33044:tid 2544] [client X.X.X.X:10178] AH00705: File 'cache/V/v/d7lKhbvN3Yp5ys4SXS@w.header' has a version mismatch. File had version: 107573072., referer: https://***********
[Thu Jun 12 13:25:43.271413 2014] [cache_disk:error] [pid 33044:tid 2544] [client X.X.X.X:10178] AH00705: File 'cache/Q/Z/adOvvbVmyA2vyg1SZQPw.header' has a version mismatch. File had version: 107605904., referer: https://***********
[Thu Jun 12 13:25:43.277419 2014] [cache_disk:error] [pid 33044:tid 2536] [client X.X.X.X:10179] AH00705: File 'cache/A/x/HZU3ybYHW4e28ERIpRwg.header' has a version mismatch. File had version: 119119040., referer: https://***********
[Thu Jun 12 13:25:43.295435 2014] [cache_disk:error] [pid 33044:tid 2712] [client X.X.X.X:10181] AH00705: File 'cache/B/b/Eyf98f952p3AQFH1_oWg.header' has a version mismatch. File had version: 102629840., referer: https://***********
[Thu Jun 12 13:25:43.299438 2014] [cache_disk:error] [pid 33044:tid 3044] [client X.X.X.X:10180] AH00705: File 'cache/J/S/GIYmUZTcfATW7MDKsSEw.header' has a version mismatch. File had version: 115598288., referer: https://***********
[Thu Jun 12 13:25:43.303444 2014] [cache_disk:error] [pid 33044:tid 2756] [client X.X.X.X:10182] AH00705: File 'cache/b/W/dJM3GIJDnbxFE@bU9B1g.header' has a version mismatch. File had version: 117763328., referer: https://***********
.... lots more
[/log]

I checked the logfiles from the last 2 years. The error entries occur for 5 days up to 3 weeks, and then disappear. Then some weeks later they occur again.

what could be the reason for that?

Kind regards
Maria Imp