Windows Server 2008R2 IIS Appcrash

We have a web application hosted on several servers of our customers. Recently on only one of them, apppool is recycled about every 15 minutes. I've checked the event viewer and there is no "Warning" level log that contains some help on any occasional exceptions. I found only some "Information" level logs with source "Windows Error Reporting" which contains contents like this:

Fault bucket , type 0
Event Name: CLR20r3
Response: Not available
Cab Id: 0

Problem signature:
P1: w3wp.exe
P2: 7.5.7601.17514
P3: 4ce7a5f8
P4: mscorlib
P5: 4.0.30319.17929
P6: 4ffa561c
P7: 1098
P8: 0
P9: System.StackOverflowException
P10: 

Attached files:

These files may be available here:
C:\ProgramData\Microsoft\Windows\WER\ReportQueue\AppCrash_w3wp.exe_a7ffe533c17adc1b839d5aa19f1af2e61b1d2_0704a515

Analysis symbol: 
Rechecking for solution: 0
Report Id: 7fc73fce-5389-11e5-9721-7824af3d6615
Report Status: 4

and this:

Fault bucket 3263327202, type 1
Event Name: APPCRASH
Response: Not available
Cab Id: 0

Problem signature:
P1: w3wp.exe
P2: 7.5.7601.17514
P3: 4ce7a5f8
P4: clr.dll
P5: 4.0.30319.17929
P6: 4ffa5753
P7: c00000fd
P8: 000010b9
P9: 
P10: 

Attached files:
C:\Windows\Temp\WERC4A6.tmp.appcompat.txt
C:\Windows\Temp\WERC523.tmp.WERInternalMetadata.xml
C:\Windows\Temp\WERC524.tmp.hdmp
C:\Windows\Temp\WERED7D.tmp.mdmp

These files may be available here:
C:\ProgramData\Microsoft\Windows\WER\ReportArchive\AppCrash_w3wp.exe_868a5e613d491401176645f115078f3faa98e9_03a912c5

Analysis symbol: 
Rechecking for solution: 0
Report Id: 4c379387-5388-11e5-9721-7824af3d6615
Report Status: 0

and also some "Error" level log with source "Application Error" with this content:

Faulting application name: w3wp.exe, version: 7.5.7601.17514, time stamp: 0x4ce7a5f8
Faulting module name: clr.dll, version: 4.0.30319.17929, time stamp: 0x4ffa5753
Exception code: 0xc00000fd
Fault offset: 0x00019fe9
Faulting process id: 0xcac
Faulting application start time: 0x01d0e79515891375
Faulting application path: C:\Windows\SysWOW64\inetsrv\w3wp.exe
Faulting module path: C:\Windows\Microsoft.NET\Framework\v4.0.30319\clr.dll
Report Id: dc73f522-5388-11e5-9721-7824af3d6615

I've tried to read the files at C:\ProgramData\Microsoft\Windows\WER\ReportArchive\AppCrash_w3wp.exe....., but have no success in understanding the problem. How may I found the cause of this errors?

You can see that it mentions System.StackOverflowException, which is quite rare to occur, so that is a good starting point for investigation. The second information you have is that it started recently.

If you do have a version tracking system, try to locate all application updates within the relevant time range and look for any kind of recursion (direct, indirect, ...). StackOverflow is usually caused by recursion in your code that went out of control.

If the code is not obvious (i.e. it is not easy to find any such recursion, or there are too many recursions in the code and you can not identify the one that is causing the problem) you should try to put logs there. Place log entries into various places of your code so that you can recognize when certain functions were entered and leaved in the execution flow.

Since you have a crash every 15 minutes, you should be able to debug this using logs very quickly, because you will clearly see the problem in the logs once you put logs in the right (buggy) function.