Reason 1: This happens in Linux if a library file named libdb-3.2.so or libgdbm.so.2 is not available or if it is available in another name. Apache Web Server requires this file for it to start. To confirm this as the reason, open the stderr.txt log file located in <AppManagerHome>/logs directory which should have the message 

"bin/httpd: error while loading shared libraries: libdb.so.3 (orlibgdbm.so.2): cannot open shared object file: No such file or directory".

Solution1: If the log file has the below error message

AppManager-Home/working/apache/bin/httpd:/lib/i686/libc.so.6: version`GLIBC_2.3' not found (required by Appmanager-Home/working/apache/lib/libdb-3.2.so or (libgdbm.so.2))

Then the library libdb-3.2.so or libgdbm.so.2, bundled in Applications Manager may not be compatible with your version of Linux. Hence, remove the file AppManager-Home/working/apache/lib/libdb-3.2.so (or libgdbm.so.2) and try to restart the server.

Solution 2: As a workaround for this problem, you can give the soft links for the libdb-3.2.so file or libgdbm.so.2 To do so,

1. Login as root user in the machine.

2. Go the /lib directory. In Linux Mandrake, go to /usr/lib/

3. Execute the command

ls -latr libdb* ( if it is for libdb-3.2.so)
or
ls -latr libgdbm* ( if it is for libgdbm.so.2)

4. The above command will return the files that are starting with the name libdb (or libgdb).

4. Check whether the file libdb-3.2.so (or libgdbm.so.2) exists. If this is not present,

-- for libdb-3.2.so

Check whether the file libdb2.so.3 or any higher version exists. If it exists then give the softlinks by executing the following command in the /libdirectory or /usr/lib

ln -s /lib/libdb2.so.3 libdb-3.2.so

If it is not present, then check for any other libdb file (libdb*). If the file exists then give the softlinks by executing the following command:

ln -s /lib/<libdb filename> libdb-3.2.so

-- for libgdbm.so.2

Check for any other higher version libgdbm file (libgdbm*). If the file exists then give the soft links by executing the following command from the /lib directory:
ln -s /lib/<libgdbm filename> libgdbm.so.2 ( eg: ln -s /lib/libgdbm.so.3 libgdbm.so.2 ) 

5. Exit from the root user and start Applications Manager.

6. If you are still having problems, contact us at appmanager-support@manageengine.com. Also, go to <AppManager/working/apache/bin> directory and execute the following command and let us know the output of the command:
ldd httpd

Note: If the host is not having the library files, copy it from another linux host.

Reason 2 : In some Linux machines, there is the problem of apache and tomcat having started properly  but when a request is made, apache is not able to communicate to tomcat.

Solution: Edit the workers.properties present in the <AppManagerHome>/working/apache/tomcat/conf/backup directory

1. Change the value of worker.ajp13.host to the hostname or ipaddress of that machine. By default this value is localhost.

2. Then start the Applications Manager. Before starting the Applications Manager, if you have previously started the Apache alone by executing the StartWebSvr.sh present in <AppManagerHome>/working/apache. Kindly stop the same by executing the ShutdownWebSvr.sh present in the same directory.